Thanks for helpin me with this folks. Couldnt do it without all of you. Heres whats up so far. I went ahead and replaced the balancer, it did look like the rubber was kind of dry rotted and trying to squeese out from between the plates... but that did not solve my problem. I drove it around some and tried to pinpoint exactly when the shaking occurs. When I first take off it seems fine, although it seems mabey a bit louder than usual. as soon as it shifts (right at about 45, 50 mph) it starts to shake violently) if I accelerate on past that , it pretty much smooths out. Letting off the gas and allowing it to slow down starts it again. If I take off from a stop and just gun it it will speed on up and go past the 50 mph mark without starting to shake real bad. Dog, by flex plate, Im not sure what you meen, and I didnt know there were balance weights any where, so Im not sure where to look.

 

And I forgot to add.. yes, popping it into neutral stops it, back to drive and in a few seconds it starts again. please tell me I dont need a drive shaft

 

kind of sounds like ujoints on the rear drive shaft. Cheap insurance would be to replace them with quality spicer joints. Pretty easy job, about 3 hours time.
Also, maybe check the front wheel bearings. Bad wheel bearings usually show their ghost at a certain speed which seem to be what you are experiencing.

 

As HeyYou mentioned earlier, sounds like you may have a driveshaft issue - either a balance issue (may times weights are welded onto the driveshaft to balance it - perhaps the weight came off or a U-joint is bad. I'd pull the driveshaft out and check it closely. If you can't figure out the issue, take the driveshaft assembly to a driveline shop to have them check it out.

As for the flexplate, it is bolted to the back of the crankshaft and the transmission torque converter bolts to it. There are weights on it if the motor is a 5.9 gas engine.

 

Actually, the whole "starts shaking violently at 45" tends to lead me in a different direction. That is the magic speed at which the TCC locks on easy acceleration. This begins to sound more like an engine miss, than anything else.

When is the last time the truck had a tuneup? Can you check pending codes?

 

Whats a tune up? Just kidding. No, the poor old things a 96 on its third motor and has more knocks and rattles and pings then I do.
The last motor was put in about a year and a half ago, and we pretty much went over the whole thing then. If it were a miss, wouldnt it do it sitting still as well as moving? Im almost possitive its a vibration from underneath at this point. and it seems that rpms have a lot to do with it. letting off the gas at just the right speed, ot when the motor shifts, or lightly depressing the gass at just the right speed, all induce the shaking and vibrating. and it starts and stops abruptly, not like it gradually starts to do it... any way, some one is comming to look at it today. Im of the opinion its something to do with the drive train, but im guessing, and I sure cant pin point it.

 

How many miles are on this '96 Ram 3500 truck (that on its 3rd motor) and where in general are you located?

 

The truck was previosly owned by a member of the military and made regular trips cross country chasing hurricanes and everything else. It was very well maintained for many years. I know for sure the odometer has rolled over at least once. My best guess is the truck has about 250k on it. If it had 350 I wouldnt be surprised. I live in florida around jacksonville.

 

had a friend come by just now, got under there and looked again, he knows at least more than I do about it, but im not sure Id call him a real mechanic. He did find that there is a good amount of movement at the center bearing where the two shafts meet up. could this be the problem?

 

Entirely possible.