Weird Vibration at 2500 RPM's and above

are the trans, driveshaft, and rear axle original to car? What I'm getting at, is this a mashup of pieces, and the driveshaft is too short, and not giving sufficient slip spline engagement? Does it have the correct unbalanced flywheel for your 360?

The engine, transmission, and rear end are not original to the car. I've checked the length of the driveshaft when i had it balanced, and it seemed to have plenty allowance. As far as the fly wheel goes, I have no idea. Unfortunately the guy I bought the car off of wasn't the guy who built it either.