Bouncing In Driveline At 3000 Rpms

[toofun]

I have a TKO-600 and a 383 stroker motor in my 69 camaro. I have been driving it for about 300 miles now and have noticed a "bouncing feeling in the tranny or drivetrain when Im cruising around at 2500-3000 rpms. It does not appear at idle or if I give it some juice and let it climb. Just when Im crusing around at those rpms. What could be causing this?? All the tranny mounts are brand new. The drive shaft appears to be about an inch or two too short but other than that I dont see any other problems. Could it be the pinion angle and if so, how do I check to see if it right? You physically can see the stick shift bouncing up and down more than usual.

Thanks in advance



Mark
TOOFUN