I've done lots of searching and haven't found anything quite like my problem. I have a 2000 GMC sierra with the 5.3 115,000 miles. The truck runs and drives great. The problem is when I roll up to a light and fully apply the brakes to a stop, complete the stop, when the chassis rocks back down from the stop I can feel slack in the brake pedal. Then if I take off really slow I feel what seems like slack in the drive train at takeoff. It does not make any noise only a bad feeling. I have had a few people look at it and u joints, rear end seem good. The truck shifts good except if I'm rolling through a turn and get back on it then it catches bad. Fluid looks good, slight burnt smell maybe? I'm really not sure where to start, drop the drive shaft?, pull rear diff cover? Hopefully not trans. Just hoping someone has ran into the same problem. Anything helps. Thanks.

 

My 99 Tahoe did this and it was bad u-joints. Took the driveshaft down and you could tell the slip yolk joints were worn bad.

 

i noticed my 1500 does this. there is some slack in the rear end itself. the rear most u-joint is new and greased, the other two are good. i found no other lose points in the driveline. no noise, but slight vibration around 45. i just balanced the tires, so its not them.