560SEC Driveshaft Vibration

My 88 560SEC has only 32K miles but it had an annoying vibration that occurred around 25 mph under acceleration It feels like a washer machine with an unbalanced load during the spin cycle. The MB Mechanic that works on my car said he was 99.9% sure it was the drive shaft since everything else checked fine. After he replaced the drive shaft, with a new one that came in two separate pieces, the vibration decreased significantly but can now be felt at 15 mph. The vibration at 15 mph is slight under normal driving but if I press the gas and hold the car at 15 mph with the brakes or go up a steep hill the washer-machine-with-an-unbalanced-load-like vibration is easily felt. Cruising at 100 mph is no fun either since it feels like there is something out of balance somewhere. MB Mechanic said he will replace drive shaft again, this time with one that comes in a complete set instead of two separate pieces, since most likely drive shaft is still out of balance.

Since replacing the drive shaft changed the speed at which the vibration occurs, is it logical to assume the vibration is caused by the drive shaft or is there something else that could cause such a problem?

Thanks for your help.