Thanks for the quick response, Larry.  Hope I'm responding correctly.
It's pretty clear that the worm shaft can't be removed by pulling the forward end bearing; the gear wheel on the worm shaft can't move forward enough to release because of the support structure of the truck.  (I'd read that the only way is to drill out the rivets and replace them with machine screws and locknuts.)  The worm gear shaft aligns properly with the gears on the truck axles, and, when rolled forward by hand, everything works fine.  When the truck is rolled to the rear -- toward the coupler end -- the gear wheel on the shaft slides back and binds on the structure supporting the shaft, and the whole assembly self-brakes.  So, whatever bearing structure exists in support of the rear (coupler end) bearing has moved or worn out.  Can only guess, but your observation that there is another bearing with a ball that supports that end of the shaft is what is creating the problem.  Until I've pulled the coupler, I'll be only guessing, but that bearing probably needs to be shimmed forward.  Too many unk-unks at the moment.  But I'll keep on it.
Thanks for the interest, Larry.
Jack (who used to live near the vestiges of the New York, Westchester and Boston RR -- "The Westchester")