Thanks for all your help gentlemen,
I've got it fixed now. On each side of the frame is a welded flange that acts as a travel stop to limit how far the mower mount linkage can be retracted. It is designed so that the lower mower link arm sits against it and the upper link arm clears it. On my tractor, the upper link arm was hitting the stop on the passenger side and actually gouged a circular path in the metal. Somehow the stop got trapped between the upper and lower arms such that the linkage couldn't extend or retract. A little bit of "persuasion" got it unstuck and the mower mount immediately drooped to the 0 position. I then used an angle grinder to reduce the size of the stop just enough to let the upper arm clear it as the linkage moves.
I had thought I could just push the whole pivot mechanism towards the side that was sticking, but there wasn't enough play, so I went to the angle grinder. Y'all's very helpful advice helped me troubleshoot and I appreciate it.
Mike