I lent my truck to a friend who said that he was driving it on the highway and it started to make a rumbling noise with a little vibration... so I drove it and it sounded exactly like the 4WD was engaged, which is noticeable above 45 mph. Probably could use some front driveshaft U-joints, but I digress.
Anyhow it did not take long to find that the t-case was stuck in 4WD in 2H. Neutral, 4H and 4L worked normally. Time to go inside it... again

At least this can be done in the truck!
Drain plug out, a normal quantity of Transfer Case Fluid came out, although it was burned brown. Sure enough, the front drive sprocket had friction-welded itself to the mainshaft. But the other journals looked normal, last year's oil pump repair and replacement felt smooth, no metal particles in the oil. I'd also welded a rod on the end of the pump bracket (the usual BW1356 mod) and it was intact. Strange.
I bought a good used mainshaft (with the oil pump in place) and a sprocket. Wasn't cheap but a fraction of the price of a reman transfer case! I also replaced the oil pickup tube with a piece of 5/16" fuel line in case it had been sucking partially shut when hot. Put it all back together, filled with 2 quarts of Mercon LV (which
is now recommended by Ford for t-cases, didn't used to be) and so far so good. Front shaft spins freely in 2H, no rumbling noises.
What bugs me is that I hardly ever use 4WD anyway, and I've been inside this t-case three times since I bought the truck. Once to replace bearings and seals, once for the spun oil pump repair, and now for a seized sprocket. At least I am getting good at disassembly and reassembly
