I have a 1992 F-250 4wd manual tranny in such new condition I can't push the 4wd shifter into 2H . Everything else is perfect, it only has 7,000 miles on it.
It maybe just too bound up, get it off the highway onto a gravel road where the wheels can slip, put it in 4 low and use it a little, then the same in 4 high, then see if it will drop into 2WD, you may have to roll a little to get it in, if it goes in I would have all of the gear boxes fluids changed, sitting around like it appears to have there could be moisture buildup and other crud that will get stirred up. Or you drain all of the gear boxes now then try the above.
Search Ford Questions