shuddering/ vibration help!

Ok here's my problem, I took my 1998 Z71 4wd into have the tires rotated and balanced Thursday. Picked it up from the shop to drive it home Friday and it starts making this awful noise somewhere between 50 & 60 mph. Sounds like when you run off the interstate and hit those bumps on the side of the road. Took it back to the shop, they said change the hub bearing. So today changed the hub bearing and put new brake pads on it. Drove it to town.. still making that noise...maybe not as loud but still there. So I dont think the hub bearing was the problem. It is not consistent, noise happens feels like vibration and if you let off the gas it stops. Anyone have any ideas? I am not wanting to take this to the dealer cause I know they are gonna run the codes.. no lights are coming on so they aint gonna find anything there... then they are gonna say we can't get it to make the noise...

Someone please PLEASE ... any help???
Thanks

Does it go away when you hit the gas or let of the gas??? What size engine do you have and how many miles do you have on it?

It goes away when you let off the gas. it's a 5.7 liter and it has around 174,000 miles on it

U-Joints or perhaps a warped Drive shaft. I have a nasty shudder on my 99' Silverado from a start and it rears it's head at 50 km/hr and really shows off at 100Km/Hr. Had the U'Joints replaced but that didn't fix the problem. I'll keep you posted if i learn any more.

It might be a random misfire, I got had the same vibration and it turned out that my heads needed to be ground down...I wold run a compression check on it and see what the readings are.

its a torq converter, i had the same problem, ended up bein my torq converter

Here are a few things you can check:

Check the tread on your tires to see if there is a cord seperating.
Chock the wheels, put the truck in neutral and jack up a rear wheel. Go under the vehicle and check the driveshaft u-joints.
Check you front differential to ensure that it is disengaged in 2WD. From your description, I would suspect this one.

Was it vibrating before they balanced them. Maybe they did a rush job balancing them and made them worse.
If there are large weights on the rims something isn't right.
bad tire or rim.

If the problem was not there before you had the tires bal and rotated I would rotate them again and see it the noise/vibration follows any tire. Remember most new problems show up after something was just done to it go back to the way it was and recheck it out.
Fireplug

Sounds like Torque converter clutch vibrating. Add some "shudder fix" supplied by a local transmission repair shop.
Tim
Trans tech 25 years

If you can't seem to find the problem in the T.C. Check the ring gear the tooth contact pattern is totally different from acceleration to coasting. On the coasting mode the teeth mesch on the covext side of theeth on drive mode it's the concave side. I had a chimped pinion gear that made a horriable sound but only on coasting as long as I had a load on the rear end the noise was non-existant. You may have the oppisote problem.