I need help Identifying a problem, That I believe to be linked to the Transmission

The truck is a 1992 toyota 4x4 pickup. With a 22RE motor. The truck has been driving fine, however recently the over drive/ drive light keeps on flashing on and off. And the gears continue to switch back and forth once it is at 45 mph or so. Today the truck did like it normally does, however suddenly the transmission seemed like it was starting to slip. I pulled over on the side of the road to look under the hood, and didnt see anything wrong. When I went to drive off with the truck the motor would rev. Up in drive buy it would only barely move. I tried putting the truck in 4x4 and it still wouldnt move. What could be causing this?