trans problem?rearend???

i have an 87 silverado and i have rebuilt a 350 and put in it and rebuilt a 400 tranny. i have sone nothing to the factory rearend. i can take off and when its really cold move but the truck rolls but doesnt go fast untill it finally like clunks. am i haveing rearend problems or is there something wrong in the tranny. oh yeah it seams not to keep up with throttle in reverse either i believe i am having rearend troubbles

I think its actually transmission problems. Sounds like its slipping (BAD). Check the tranny fluid just for $#*^$ and giggles. Be sure vehicle is at operating temp, then while firmly holding down the brake pedal pop it into gear (any gear wil do) and hold it for about 1 minute. Transmissions warm up real fast but only when they are in gear so this will help insure you get a fairly accurate reading. Once you got motor and tranny warmed up, park on a level surface and put in park. Then check trans fluid (a few times). Should be inbetween the hot and cold lines. Although it sounds to me like a more serious transmission problem, its always worth checking the simpilest solution first.
Doubt its your rearend because rearends are nothing more thamn solid pieces of metal (gears) constantly in touch with each other whereas a transmission (automatic) is a series of bands that give a little to allow for a smother shift between gears.

sounds like someone blew their clutches to me. Should have checked your fluid more often. Don't worry it happens to the best of us.

This also might be a plugged up filter or a broken o ring on your filter pick up tube.Dont run it till you ck it!!Sounds like it might be to late anyway