Car's an 83 300D w/194K. Over the past couple of days I've notice a distinct shudder/vibration (not heavy but definitely there) from what seat-of-the-pants feels like it towards the back of the car. The problem starts around 45 mph and seems to pretty much vanish at about 75 or so. Prior posts are going to have me looking at the flex plates. Is there a center bearing on the drive shaft? I felt the wheels after a long drive and none of them are hot or smell bad so I don't think it's brakes dragging. Tires are in good shape. The vibration feels too high a frequency to be a wheel balance problem. I installed a pair of re-built AutoZone axles in the car spring of 2004. Boots still are OK. I am not sure of the quality of the rebuild but eye-ball check didn't see anything out of the ordinary.
I'm not into doing a lot of work for nothing. I am asking what would be the most logical area to begin looking for this problem. Can the flex discs be bad while not looking as if there are any problems? Dus to time constraints I may need to have the car worked on by an indy. The flex plate job doesn't look real hard especially with the car on a lift. 2 to 3 hours work??
Advice sought and greatly appreciated.

Had the same problem. Replaced bad center bearing on the drive shaft fixed the issue. The bearing had separated from the mounting bracket. Not too hard to fix. I posted the steps I took in replacing the bearing.
http://www.peachparts.com/shopforum/showthread.php?t=102521

rmmagow,

Check the mounting bracket that holds the center of the drive shaft. The rubber gets old and cracks. only fix is to replace the bracket and its not too expensive. Labor will be much more than the bracket but its a DIY project.

You should be able to determine by visual inspection any flex joints that are worn and need replacement.

P E H