[FC] FC Power Glide - Lost Reverse

[Valiant Effort]

I lost reverse gear with absolutely no warning symptoms on my 61 Greenbrier
(PG) while traveling from Canada to Texas the other day.  I stopped in
Memphis for the night and noticed I had no reverse.

One minute working like new, the next.... nothing happens when the lever is
moved to reverse...
Not even a minor change in eingine idle.

Tranny works perfectly in neutral and forward gears.

Any ideas on what may be causing the problem, and how I might troubleshoot
this, would be greatly appreciated.  Let's start with the easy stuff first.

Thanks,
Chris