pop or clunk sound 50% of the time when going from forward to reverse or vice-versa

1993 GT convertible - Slow speed in driveway - 5spd - all rebuilt 5k miles ago w/ new Centerforce dual-friction clutch, rebuilt rear end with extra posi clutches, etc... I checked U-joints by wiggling - no play, motor mounts - tight and no cracks, trans mount tight and no cracks and attaching bolts, etc... looked for exhaust bumping in areas (none seen), and so forth. I put the car in reverse to back it into the garage after driving forward and let the clutch out easy - the car moves as normal 1 foot or so and then a pop sound can be heard and felt through the floor - no other noises or funny things going on. It will also do this when going back into 1st gear - ease out on clutch and go about 1 foot and there is another pop and can be felt in the floor. It does this about half the time. It has never done this before - I have rebuilt everything (total engine out/rebuilt, 5spd trans out and rebuilt, rear out and rebuilt with extra clutches, subframe connectors, etc...) 5k miles ago and it just started doing it 100 miles ago. I do not dog the car - but do accelerate briskly on occasion but the problem did not show up after any particular incident or hard use - just showed up one day after normal use for apparantly no reason. Any ideas? Are the extra cluches in the rear end causing this? thanks....

car still popping - still can't find out why - would a clutch do this? internal transmission problem?

good ideas - I wiggled the drive shaft the other day but it has no slack, no loose u-joints, I will check the 12mm bolts to see how tight they are - seems logical that they would cause it based on the feel and the sound.

checked the 12mm bolts on the drive shaft - they are tight - I also tried wiggling the drive shaft again - this time there was NO slack in it - on other occasions I have been able to wiggle it back and forth a half inch or so - is this normal? I always have had some back and forth drive shaft movement on all the cars I have checked - maybe it is because it is a 5spd and parked in gear with the wheels on the ground and parking brake set? Nothing looks odd under the car - just your typical slight seeping leak from the rear transmission seal by the yolk....

Do I understand you correctly -- I am to pull the drive shaft and put grease inside the front yolk splines and slide it back on the transmission tail shaft? That seems to make sense - it would make the drive shaft slip in and out smoothly as the car goes over bumps or goes from forward to reverse. Question --- doesn't the transmission fluid lubricate the splines as you drive?

Great! - now I DON'T look forward to getting down and dirty under the car again with no lift but at least it is worth a try.... Sometimes I wonder why I love these cars soooo much.... I could always buy a new Toyota and be bored hmmmmm...... Thanks for the advice.