First, I would NOT drive the car much into the "vibration zone." We can only imagine how bad the vibration is from your description.
Second, determine if the vibration is actually before/ after the engine / transmission.
Try bringing engine RPM up in neutral. If not vibration, jack the car up SAFELY in the rear and confirm it vibrates in gear.
Next, remove the rear tires and repeat
If the vibration is still there, try different gears
Try fairly high RPM with shifter in 1, then 2, finally drive
Last, remove the driveshaft and try different gears.
IF these tests point to the shaft itself, pull the U joints completely apart and inspect them, for bent/ damaged/ missing rollers, rust, lack of lube, missing/ improper installation of snap rings, etc
Also examine carefully how the joint fits the rear yoke saddles. It should fit fairly tight, with NO side/ side play