My 1995 S500 sedan has a terrible vibration in the car and steering wheel that starts between 50-60mph. The transmission was replaced 18 months ago and the tires are new and have been balanced. In addition, the car was recently aligned. I am not sure whatelse it could be but I have viewed theads suggesting everything from transmission issues to broken motor mounts.

 

now with that new transmission, did they give you new mounts and made them tight like they suppose to?

50-60mph vibration can be number of things for real. Anythign from one loose bolt in the suspension set up to an off balanced drive shaft, improperly torqued flex disk bolts to old steering shock.
I would start poking around the front end suspension first as it's most tedious yet simple to diag. once you locate the looseness. Then check mounts, then check to see tranny instal set up. was the bolts of the flex disk and drive shaft (in places it was removed) retorqued after 100 miles or so or 3 months after the work done. Are the bellhousing bolts correct torque spec. and there.

 

defenetly front suspesion

 

Upper and lower control bushings will cause a major headache. Also check the ball joints. It is very hard to find play sometimes. even so they are usually the culprit of a vibration at 50 - 60 mph.