A mechanic friend of mine wonders whether the dealer replaced the standard open diff with an LSD (the dealer claims not.) He seems to think my description of the sound/vibration sounds like the loaded rear wheel fighting for traction via the clutch pack in an LSD. Does an LSD make a noise or vibrate strongly when loaded? and why does it only happen when everything is hot (i.e. after a decent run)
Your mechanic friend is right. An LSD can make groaning and clunking noise (when turning) if the wrong differential fluid is added or the proper friction modifiers are not added. It can tend do it more when it is hot. You can find out if you have an LSD by jacking up one wheel with the transmission in neutral and trying to turn the wheel by hand. The wheel should rotate very freely without LSD and be impossible or very difficult to rotate with LSD. Try both sides so as to maybe detect something else that is the problem.
I have rebuilt a few Chrysler differentials. There is really not much in an open differential to make noise like you describe. When side case gears shafts wear out there can be some clicking or snapping when coming to a straight stop. When the side case gears begin to disintegrate there can be heavy clunking and grinding and can jerk or stop the car when turning. If the side case gears were not properly shimmed I don't know what that would sound like - never done that.
Best advice I can give you is to safely get one or both rear wheels (safely) off the ground and start rotating wheels while feeling and listening. If the noise is coming from the differential, might have to pull the cover to have a look and see what parts fall out.
