I have a vibration somewhere which sometimes seems to be worse under heavy load and is worse at different speeds. The tires are balanced and it seems to be worse when the clutch is engaged, so I assume it is related to the powertrain. Also, there is occasionally a clunk/pop that occurs when engaging the clutch. Here is some history to the problem. All bushings are poly and in good condition. I just recently replaced the center support after it failed in less than 2 years (must have gotten a really cheap one last time :( ...) (this helped reducing the amount of the pop/clunk but it didn't eliminate it completely.) I had to swap out the M47 transmission earlier this year (though with one of a similar age, due to a broken gear on the input shaft). I thought the transmission swap initially helped the vibration problem, but it seems to have come back. The engine and transmission mounts are in good condition. The u-joints on the driveshaft appear to be fine but are original (281,000 miles). Most recently, I put IPD sport springs and adjustable torque and track rods on. The pinion angles are pretty close. I think the new spring setup actually makes it easier to feel the vibration as it seems to have intensified after the upgrade. I have put over 1000 miles on it since, and it hasn't changed or settled in much.

My assumptions are that the vibration is either caused by the driveshaft being out of balance, or something in the differential. Has anybody had any experience balancing a driveshaft to help solve a persistent vibration? What would it cost to have a shop balance it/is it worth doing? And if so, should i replace the u-joints first, even though they appear to be fine? What are the chances it could be the differential?

Thanks for any ideas and advice!