Update:

OK, Changed the differential fluid (Mobil 75-140 synthetic) and added some LS additive, and now the speedo works, but the OD light is still flashing. I disconnected both negative battery terminals, and I'm going to let it sit for about 15 minutes to clear any codes and see if the OD comes back on.

Everything seems to work fine. Even with the OD button flashing and the tranny in overdrive, when I push the OD button, it shifts back into 3rd. Push it again, and back to overdrive.

Maybe clearing the codes will solve all this...

Thanks,
Stacey