Driveline vibration out of now where! Help!

Went out and tinkered with it a little after work. I still have it supported on jackstands under the rear frame. I removed the brake drums just to be extra sure it wasnt them. Still the same thing. Next, i put a floor jack under the rear chunk so i could raise and lower the whole axle to change angles..It didnt make it change any. Last, i put a 2x4 between the seat and gas pedal and got it to the rpm it does this the worst and looked up under the car with a light, both at the rear and at the trans..The driveshaft spins just as straight and smooth as can be!..So i sat in the car and played around with it..It starts at 1400 rpm and ends at 1900-2000. It is barely noticeable in 2nd gear at these rpms but very bad in 3rd gear at these same rpm's..After 2000 RPM it smooths right out..I really dont know what to think at this point. Transmissions arent cheap and i really hate to buy one without more evidence that the trans is the problem for sure..I just dont know..