Weird Vibration at 2500 RPM's and above

Ever since i bought this scamp it has a vibration above 2500 RPMs (no matter which gear you're in). I've put new wheels and tires, had the driveshaft balanced, new u joints, and put in a new transmission mount. I still have the same vibration. The guy at the shop that balanced my driveshaft told me the slip yoke looked like it was wore out a little bit and I may need a new tail shaft bearing.

Sometimes if I accelerate rapidly it doesn't vibrate, if I get the car about 3500-4000 RPM's it doesn't vibrate, or at times if I'm cruising down the highway, if I pop it out of 4th gear, and put it back it will stop vibrating. Does this sound like a bad tail shaft bearing?

I know this is one of those issues that you have to keep throwing money at until you get it figured out. But I would appreciate any advice I could get.

The car has a 360 4 speed for reference.