Wellllll, here's the update. (Sunday evening, 5/8)
He was able to get it back together with a lot of cussing, wiggling, pounding and so forth from him and some help from people pushing and pulling on the wheels, wiggling the jacks and so forth. I guess the tractor realized it was outnumbered and gave up - it went "click" and slid together - it was hanging up on the pilot bearing as suspected.
It appears I was barking up the wrong tree and I was wrong about the diff lock pins, and quite frankly, I am glad that I was wrong. He doesn't need any more expensive problems. (I still would have looked inside the rear cover, just to be sure, but that's me, and it isn't my tractor.)
What seems to have been happening is that the front coupling was so worn as to be ratcheting under load, intermittently skipping teeth and grabbing, and the noise was transmitted to the rear end, making us think the problem was there, and it was not.
He replaced the front coupling and front drive shaft, an oil seal in the bell housing and the utterly worn out tie rod ends (and that expensive microswitch). He did disassemble and look at the front drive axles, some bearings and checked the front differential, and they are all fine. The steering components, as MHarryE mentioned above, were very dry and the seals were shot (replaced). Fortunately there were no metal "sparklies" or chips in the oil, and no water had entered to make the oil milky.
He's out mowing, and I *think* he's OK now. I'll keep my fingers crossed that my suspicions were unfounded and that he's back in business.
As the Bard said a long time ago, all's well that ends well, but I doubt he was talking about tractors ;-)
Best Regards,
Mike/Florida