Do you have oil in the transfer case and tranny? Did you try putting it up on jack stands and running the wheels. I'd recheck for play in both diffs and fluid as well. My initial guess would be drive shaft or ring and pinion Esp with the decel symptom. Did they REALLY swap the dif? While you are down there, check the front CV's/Birfs and the Tulip joints. After that I'd start looking at bearings. If it is only making sound when you are moving, it's gotta be behind the tranny.

Lars,
Gold Coast Cruisers |My 4Runner