Eric,
I have an 88 GL and had a very similar sounding problem with my
speedometer. Assuming our speedometers are the same you might try taking
out the drive gear at the end of the shaft that holds all the numbers
(the one that falls off before the numbers), apply several thin layers of
rubber cement to the inside of it's center hole (can't use glue because
it will glue the case or other numbers). let it dry well and after you
have lined up the numbers force the last gear back on. Mine has lasted
for more than a year now.
In addition, my trip tick numbers were the real problem. They seemed to
jam and force the upper numbers to go off. So in addition, I had to
remove a tiny gear connecting the upper drive gear (the one I rubber
cemented)and the lower trip tick numbers. That may not be your problem
but keep an eye out for it. My problem started with a reset of the trip
tick numbers to 0 and got worse in time.

Bill Hayeslip
88 GL
Pgh,Pa.