With the information you've given, it seems unlikely that the problem is the rear driveshaft. You did install a new U-joint in the rear of the rear driveshaft when you got the CV joint installed and the shaft balanced, right?

It could possibly be a pinion bearing in the rear differential. Could be the transmission/t-case mount, if you've already checked the engine mounts.

Could be an out-of-balance tire. Could be a loose wheel lug nut in front or rear. I'd chack all that stuff.

If you still can't find it, you might want to put the vehicle on a hoist and run the engine with the tranny in gear and see if it does it then.