General Maintenance & Repairs - driveshaft vibrating?

Also having another problem. Had to replace the rear input shaft seal where the driveshaft goes into the transmission because it was leaking. I just bought the car last year and when changing the seal I noticed there was about an inch and a half at least showing between the ds and the inputshaft, from what I have read it is supposed to be about 3/4. It is a dennys ds and there is bad vibration coming from that area starting at 30mph. The vibration seems to go away a little bit under acceleration. The car is a 1999 trans am a4 cam and headers, and if I put it into first gear the vibration is real bad. Is my driveshaft out of balance? the shaft has a little bit of play, how much play is normal? any ideas? thanks IA

Could be the bushing in the tail shaft.

I had this same issue except it happened when i took and had my tires balanced at firestone. *biggest mistake of my life* After fighting and digging about this random vibration i noticed the balance weight off the pinion was missing. Swapped that and no more vibe.