have someone hold ur driveshaft steady when u rock the wheels , if theres alot of movement in teh rear b4 u hear the bang its most likely rear end slack

other than that its a u-joint

 

Yeah, I mentioned in my 2nd post I can turn the driveshaft with the tranny in Neutral, about an 1/8 of a turn before the gears grab when I turn it in opposite directions and it does clank some...maybe the poly tranny mount just amplifies the sound in the car too..that will be replaced next week when it arrives.
I need to put one of my other cars up on the lift and observe what kind of slack they have, I don't think 1/8 of a turn is that much from what I remember.
I'm going to go ahead and change the u-joints, probably just take the drive shaft somewhere and have them changed and the driveshaft balanced all at once...at least rule that out and then move on to the gears if that doesn't cure it.

 

Ok, I did the same wheel jerking test with the GN and the 99 TA, both have about the same amount of play but neither make the banging noise in the rear that the LT1 is making when I do the same thing.
I put a stethescope on the u-joint and the yoke and the differential, its definitely loudest on the rear u-joint, so hopefully thats it although nothing can really be felt when you hold it while jerking the wheels back and forth with the tranny in neutral. Could be the gear slack and the noise is propagating out of the diff thru the yoke and to the u-joint I guess... U-joints get changed tomorrow then go from there...

 

Ok, changed both u-joints, and the problem is still there.
I got going about 70mph and put it in neutral and shut the engine off, just coasting and there is quit a thumping noise under the car...
Getting a convertor put in week after next, I'll have them check the tail shaft play then....

 

Sounds like you have a Torsen rear differential noise. Does it make a similar moise when slowing down and turning? Does it do it both hot and cold?

 

Yep, I think your on to Daddyss, was just at my frineds shop and we took the rear cover off and there's quit a bit of backlash slop between the smaller pinon gear and the bigger gear and the noise is definiteyly coming from there and also maybe from the axle side gears..whatever they are called..the smaller ones on each end of the axle, but not near as much slop there.
The LS1 tranny mount I put in last night really softens up the noise inside...guess its a band aid until I can just replace the entire rearend with something stronger.
The tranny tail shaft also needs a new bushing to get rid of the play/slop there as well..that will get fixed in a few weeks.