Update:
Bought the used '01 Explorer rear (4:10) for $300. A nice clean southern truck, so no rusty parts. Got 2 Qts Valvoline 75W140 Synthetic gear oil (on sale at AZ) for $18. Got the rear home, stripped it down, cleaned it out, all the gears look great, even the oil look like it had been changed recently, removed the disc brakes, swapped over my drum brake parts and reinstalled it under the truck.
Tomorrow I'll install the new axle seals, get the axels back in it, put the new brake shoes on the driver's side, clean up the drums and reinstall them, fill-er-up with oil and finish putting everything else back on. I should be ready for a test drive by late afternoon. I sure hope this rear end is nice and quite. I believe it will be.
Once this is done I can get back to my list of other things needing attention.