Get the driveshaft inspected to see if it is balanced. Inspect the U-Joints.

 

This is going to sound stupid but has any of the tires ever gone flat on you, and have you ever used fixaflat on it ? also if they go flat and it rains water will get inside the tire and when you inflate it, the water usually stays in. I actually had this problem once with my old dodge, i'd be goin 30-50 hit a bump and i swear the whole truck would shake like an earthquake. It was because when i'd hit a bump it would knock the water/chunk of fixaflat loose and it would unbalance the tire untill i came to a complete stop, then it would be fine untill i hit another big bump. Believe it or not it did it for 2 months and i had those tires balanced twice and finally i just got new tires and when they pulled the old one off the rim there was a buttload of water in the tire and a big chunk of fixaflat. after that it never happend again.

Not saying that's your problem but just throwing it out there..


Edit: forget everything i just said i missed the part where u said u got new tires.

 

I have a 1994 4x4 reg. cab. I was having vibration problems only during highway driving unless I had weight in the bed. Studied this problem for some time, even taking it to shops, new tires etc. One day I noticed it wasnt happening at the same speeds if I was on blacktop. Interstate with seems cut in the concrete-vibration, smooth blacktop-no vibration. I have read since then about stiff suspensions and a certain length of truck on uneven highway causing highway oscillation. (spelling?) Seems that some older fords had a severe case of this problem. Just a thought if your truck does this on all types of roads at the same speed????

 

IMHO, I think the road surface has alot to do with it. It does on my truck anyhow!

 

The other day I jacked up the rear axle with the rear tires off the ground and brought it up to 41 and then 49 where it's shaking badly. My new tires are moving side to side and back and forth at those speeds. I think it's the rims because it did the exact same thing when I had the tire company look at it before I put new tires on it. I also tried it with the rim off to see if I had a warped axle or brake drum. The truck didn't shake at all with the wheel off and my dad checked the axle with micrometers to see if it was moving back and forth. It looks as if the lugs are oval shaped and letting the rim mount correctly but then move. Do alloy wheels warp?

 

are both rear tires shaking?
how about the fronts?
can you identify any that don't shake?
are the wheels stock or aftermarket?
if the lug holes are wollowed out, they might be going onto the axle offcentered. that would explain some up/down bouncing, but it wouldn't explain in/out movement.

 

I had the exact same vibration problem from 45-50mph, and couldn't find anything wrong. I just cracked one of my rims the other day, and switched back to the stock ones, and got new tires and the vibration is gone. The ride is a lot smoother too. My other rims had a different backspacing and stuck out a little, which I liked the look, but these ride a lot nicer. Just my 2 cents.

 

I don't know about the front because it's 2wd. I'm leaning towards more of the bent rims theory because I've bought new tires for it already. I'm looing into rims for it now. I've found some proline alloys at pepboys for like 70 bucks a piece. What backspacing is stock, 4.5?

 

Any updates? Did you find the problem?

 

If your wheel was bent and they didn't notice it when they balanced it, it's not much of a tire shop.