Here's my experience...
This happened on my 98 AWD Explorer at 110,000.
Not as dramatic as your situation but very similar and was able to resolve it completely.
The rear end would chatter. The whole truck would shake. I had no clue what it was. I actually thought it might have been something on the front end because it only happened during low speed turns. The dealer told me "I needed to replace the entire rear end", I was like, okay give me my truck back please.
I did a lot of research on the explorer forum (and posted about it). I drained the rear end diff, added Amsoil gear oil and their "non-slip" additive that goes in diffs, it comes in a separate very small bottle. After a few hundred miles the chatter went away COMPLETLY! And the differential was 100% fine until I sold the truck at 215,000 miles.
It would def be worth your time to try this. Will prob only take you less than an hour and maybe $100 in "parts".
I still have the products I used and can list the Amsoil product numbers when I get home on Saturday if you need them.
In a nut shell, you remove your rear diff cover. All the fluid will come out.
Clean the surfaces well. Apply proper gasket to your cover. Reinstall and properly torque. Add the gear oil. Then add the additive and reinstall the full plug. There are lots of walk throughs on this and how to properly apply the gasket on the web and YouTube and stuff.
2007 XLT 4.0L Super Cab 4x4
Last edited by B737; 02-15-2012 at 06:11 AM.