Drive shaft collars slipping

first time ive run into this. lately, the driveshaft has been slipping on the collars and getting loose and sloppy. hasnt been a problem before.the collars are super tight and relatively new.any ideas? the shaft has 1 collar in front and 1 in rear to keep about 1/8 to 1/4 of front to rear clearance. does anybody make a collar with the splines of the shaft on them on the inside? the car has normal couplers and female splines in diff. should i put 2 collars on, or is something like the trans moving around too much causing the shaft to be pushed backward? car lost a driveshaft during a run, even though it was checked before the run and was tight and adjusted right.kinda lost on this one. it is a funny car with blown hemi and 2 speed glide .

 

We had a TAD car that started doing this. It spit the coupler out once and lost the engine (over rpm'ed it). Found that the engine clamps were lose and the engine was moving back at the hit, then would move back into place on the shut down. When we went to service the clutch, we would have 1/2 of clearance in the lock rings. We got the engine tied in and went to 2 lock rings on each end. Way cheaper than engines.