problems help!

well i get to like 2 grand in overdrive its a bran new tranny but in my rear end i get a lod granding noises and really bad vibrations anyone have a clue on what it could be i think the u joint or whole rear end plz help :banghead:

First I would try to isolate the problem. Get out of OD and run up to the same speed as you get the noise in OD. (If the noise in OD is at 60mph, go 60mph in 4th.) If it is a 6 speed, use fourth, since both 5 and 6 are OD. Do you get the noise? No - it is probably the OD. Yes - it is probably the U-joint(s). Did you just change the gearbox? Was it making the noise before the gearbox was changed? Was everything installed correctly - bolts tightened etc?