vibration above 30 mph

Heres what i have: 1991 S-10 2wd, 4.3L, 700r4 auto. My problem is something starts vibrating pretty bad when driving above 30 mph. The trannsmission shifts fine and there are no abnormal noises other than the vibrating when driving it. I bought the truck for 250 bucks and it needed a few things. The most major being a crossmember for the tranny because it was rusted in half. Other than the crossmember being completely rotted out everything else seemed to be ok, nothing to be too overly concerned about. After i replaced it the vibrating wasn't quite so bad but it's bad enough that i don't trust it.If anyone has any idea of what it could be PLZ PLZ PLZ help me out.:banghead:

Wheels balanced? Got any bent rims? Bad Tires?

the tires on the back are bald but they are balanced. a friend followed me to his house where we are working on it and he didn't see anything out of the ordinary. he seems to think its the rear-end or a u-joint because if you drive down the road and put it in nuetral it doesn't vibrate at all, only when power is applied

Probably the front "U" joint. Front joint will start at approx 30mph, rear joints around 70mph.

im hopin its not anthing to expensive. ill try the front u joint first. thanks alot guys

Check it for looseness first, don't replace it needlessly.

everything seemed tight with the u-joints. the rear-end didn't seem to be makin any wierd noises. Maybe the bearing in the tail shaft?

the problem was the rear u-joint didn't have the clips that hold it in after you tighten down the clamps so the cups were about a quarter inch out of where they should be. And it was missing 2 needle bearings. apparently the guy that owned it before me was a genius and decided that it didn't need them. PROBLEM SOLVED!!!! Thanks alot for everyones help!!!!!!!!

Glad you found the problem. I love happy endings.

Thanks for reporting back - most don't.