I really wish I could tell you what is wrong, but that is virtually impossible by simply reading a description. The ONLY way you will know is to start turning, shaking, prying or whatever until you find it.

As mentioned by a previous poster, start by getting the rear wheels in the air safely so that you can rotate everything and find the culprit.

I have maintained right at 1 million miles of 240D's and probably a little over 100,000 miles of 300D's personally owned and have NEVER had to touch a rear axle shaft for failed boots, joints or ANYTHING. I will be surprised if you find the problem there.

It could be something as simple as running over a piece of wire and getting it wrapped around the driveshaft or one of the axles.

Keep us posted,