When I bought my '03 1500QC Hemi last October, I had the same problem. There's a thread on this bulletin board somewhere that topped 35 or 40 posts. I never had this problem with my '95 short-bed Ram, either.

Being an engineer, I obsessed for quite a while as to the actual cause of the vibrations. I found that they only occurred on certain freeways that were constructed in sections that were 12' long, so there were seams in the concrete that were perpendicular to the direction of travel every 12 feet. I also found that the shaking only occurred between the speeds of 65 and 75 MPH. Faster than that, you get a smooth ride. Slower than that you tick everyone off around you.

I took some measurements on the truck. Nothing really precise, or anything. Just with a tape measure. After doing some math (I had to replace the batteries in the calculator I hadn't used since college), I determined that the bouncing is due to frame oscillation (or flexing). Now, the frame will flex in the middle normally and they are minute and very short. The problem really begins when you're at speeds that are fast enough so that you introduce a new flexing moment to the frame before the old one has stopped. So basically, you hit the first seam in the freeway, the frame starts to flex, and as the bend is at it's most-extreme, you hit the next bump. This essentially amplifies the flexing of the frame and makes it more pronounced to passengers in the cabin.

I tried everything to get the bouncing to stop. The only two things I could make work were:

1) approximately 100 pounds of weight in the bed, right against the tailgate.
2) a shell or hard tonneau cover (i.e. a SnugTop or *******)

The first method introduces a bit of weight to the back end of the frame of the truck. It basically acts the same as when you pluck a guitar string and then put a finger on it, dampening the oscillation. The crappy thing about this is that the more your truck weighs, the worse the gas mileage you'll get. It does work, though, and this is the way I've gotten my truck to stop bouncing. I added four bags of lead shot.

The second method involves spending between $750 and $2000. The shell or tonneau do two things: First, they add weight to the frame of the truck much like the first method, only distributed along a greater length. The tonneau cover also (in theory) allows you to take advantage of a little aerodynamic down-force from the air flowing over the top of the cab and down. Without the tonneau, the flow simply strikes the top of the tailgate. The down-force thing is small, for sure, so I think the prime advantage is the weight added. I have not tested this method personally, but my next door neighbor has an '03 QC Hemi like mine and he put a shell on it, saying the vibration problem went down significantly.

I'll tell you what WON'T work, now. Take it from a guy who spent time and money trying to figure this out. Don't spend time and money changing these things.

1) different shocks
2) different springs/spring clamps
3) different tires/tire pressures/etc

The only other thing I would have liked to try is strengthening the frame longitudinally (front to back). I went to the dealership to look at a truck that was up on a display stand, but I couldn't find any suitable attachment points for frame modification. I was also told that this would absolutely void my warranty.

Now the GOOD news!

Memorize this post. Recite it to the next cop that pulls you over for speeding. If he's like the one that got me on the 118 eastbound near Balboa Ave., he'll stare at you for about 30 seconds with his mouth wide open in confusion, then he'll tell you to get out of his sight. I think I confused him enough with this so that he forgot to write me the speeding ticket.

The other fun thing about this is that when the truck bounces a little bit, my 5 month old daughter has figured out that it makes mommy and daddy laugh hysterically when she makes noise while getting bounced around in her car seat. Funniest thing ever.

 

If what you say is true and the frame is flexing enough to feel, wouldn't it, the frame, react differently if the shocks were changed to at least reduce to amount energy that is being tranferred to the frame. I can't argue the fact that the frame flexes to some degree, but to an extent that you can feel while riding in the vehicle....

I'd love to see these calcs.

 

You'd think that any change to the way the energy is transferred to the frame would change the way it bounces. As to why I didn't see a change with shocks or springs being stiffer or softer, I'm not sure. I wish I knew all the answers to this issue, because it really is the only thing I don't like about my Ram and I'd really like to make it go away.

 

i own a 04 2500 long bed reg cab 4x4 with the hemi and 5 speed od. there is definitely a problem here. i lowered the tire pressure to 40 frt and 40 rear when the truck was empty and boy did that make the hopping worse(tire are the bf goodrich all terrain type), i had a dodge mechanic in the truck to prove the problem. i usually drive 80 on this strech of highway and when i entered a bridge the truck started bouncing like the tires were basketballs, when i slowed down it didn't stop until i was doing 5 mph when i pulled off the road. FREAKY. the mechanic inspected the front end and found nothing to take exception, he then put the recomended tire pressure what is on the door (i think 65 frt and 80 rear) , we then took it for another ride on the same stetch and the hopping did not happen, but it feels like it wants to. has anyone else experienced the same as i have described? the truck did this from new and it now has 8k on it. by the way the engine and trans is awesome as are the 4 wheel disk brakes

 

RJ, great response. Thanks for putting some time and I assume some money into this.

I had lowered the tire pressure and it did help. Originally the factory had the pressure at 35lbs all around, I dropped the front to 32 and rear to 28. It cut the the vibration down by half. I had a feeling is was some kind of resonance, what you say makes sense. But it does seem to reason that if you dampened the hit to the frame it should lower the resonance, or at least slow down the build up.

What shocks did you try on you truck?

 

How much flex are we talking about here?

RJ. Perhaps it was a combination of shock settings and spring rates that hadn't been tried.????

 

2 or 3 60# bags of Quickrete for ~$10...

 

I don't have to worry about rain getting in those bags.

4 bags of playground sand #40lbs also $10 plus tax. Just load it by the tailgate. I have a rubber bed mat so it doesn't move much.

 

Spring clamps on the rear leaf springs will solve the bounce until you buy new shocks and they only cost $10 at AutoZone...the factory shocks are complete junk and should be recalled for replacement because they make driving at highway speeds dangerous.

 

The shocks do suck. I took mine down a dirt road that was a washboard. Several times it was all I could do to keep it from going into the river.