Both have finished walls that are inaccessible. Does anyone have any
clever tricks for getting these goddamned things back on their tracks?
I've looked at them, scratched my head and come up with zilch so far.

I had a similar problem.

The links and pics posted by previous posters show pretty high-end
trucks and tracks. On mine, the track was a single rail and the
trucks had a single wheel. I got the door off by taking of the trim,
but there was no way to get the track off and a new one in. I ended
up cutting through the drywall on the back side of the wall and
installing high-end tracks and trucks.

A contractor friend suggested that I try using a car jack or something
similar to spread the pocket as much as possible to get access, but it
didn't work because there were cabinets on both sides. It might work
if you have open walls.

-Zz