I think I know what you're describing. If it's what I think it is, I had the same problem and thought that it was the U-joint.
If it's a click or pop when you're going into D or R or you hear it when you're driving slowly and suddenly let off the gas, the axles need servicing.
It's a common issue with our Ms and not too costly to fix... all labor and no parts necessary.
Check out my previous post on the fix:
post6409138.html?hilit=splines#p6409138