1983 240D 45-50 mph vibration.
 

I've looked this up and have seen a few potential causes.

The vibration is pretty bad, the whole car shakes if I stay at that speed. Once I'm above it things are smoother.

The right rear drive axle has both boots cracked and dried so I assume that's the cause. I plan on replacing it with a manufactured one from CVJ Axles in Colorado.

Is there anything else I should do while I'm at it? Is there something I should try first?

Axle boots don't cause vibration, but if they're cracked, they'll start leaking and then the CVs will start clicking and then bad things happen - so go ahead and change them. But, I'd look at your center bearing and flex disks as primary suspects. Tires are also right up there too. Then start down the path of looking at anything else that rotates. Since it happens at a specific car speed rather than engine speed you should be able to rule out stuff in front of the transmission...but maybe not.

Ok. I have the flex discs and center bearing ready to replace anyway.

If the boots are cracked and dried, that means there is no grease in the CVs. Yes, that is your cause.

Possibly, but not likely. When I drove my donor 123 home there was no vibration, clicking or anything indicative of a CV issue. During the course of the swap I looked at the axles hoping they might be annular ones I could use on my 107 and both boots were not just cracked, but separated into two pieces. It had obviously been driving that way for a long time!

If the OP replaces the axles, center bearing and carrier that will fix the problem...unless, as already stated, the problem is with tires in which case that's an easy fix. Have them rebalanced as part of getting things dialed in.

Something is falling into resonance at that speed. You just have to look at the drive line from the trans yoke backwards until you find the cause. Since you see the axles are compromised they require replacement anyway. If you have the carrier bearing install in as well. While doing those jobs look for anything that is sloppy or worn out and replace it. Also look at all 4 tires for missing wheel weights.

Thank you all for the info. I'll switch wheels to see if that changes anything. When I get some free time I'll have at the drive line.

Many years ago when I had a W114, I had the exact same symptoms and it was the center bearing. Replaced the tires first to no avail (although they needed replacing anyway). Not saying that's your problem. Just sounded familiar.