'O/D Off' light flashing - need advice!

[2fast]

The 'O/D Off' light on the instrument panel is flashing. The Manual says "...If the O/D off light is flashing, this indicates a transmission problem. Take your vehicle to the Toyota Dealer immediately".

Any ideas on this folks? What causes it? How serious?

I have a 1990 4Runner with 4WD and an automatic trans. Only 37,000 mi on the clock.

Thanks.