Re: 04 Navigator noise @ rear
Rear ends can vary (uh huh!), depending if you want a locker, posi, open diff (standard, and generally the lowest cost). It's a somewhat complex procedure in order to get the gears to mesh correctly when you install. There are rebuild kits available, from Randy's Ring & Pinion or Yukon Gear. You'd have to check their websites for your specific application of course. $2K seems high for only one axle. $1700 would cover BOTH axles on my Durango at a gear specialty shop. Prices can vary, so be sure to shop around. A lot of performance/customs shops do gear swaps/replacements at a lot lower cost than the dealer, and can sometimes last longer since their staff can have more experience over dealer personnel.
New technology can come at a price as well, by this I mean if your Exp is equipped with that RSC, or advance trac, which might require computer calibrations once the gears are installed so the new gears don't make the systems to crazy. Again, $2K is still too high for only one axle. To replace the axle gears in my Dodge Durango, the AVERAGE price is roughly $750 USD.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!