1988 1500 won't shift out of low

I have a 1988 1500 1/2 ton 4x4 with a 4speed automatic overdrive, the 700r4 i believe. I had to have it rebuilt about 2 years ago. It was fine until yesterday, i went to leave, and as soon as i left...it decided not not to shift out of low gear. I checked the fluid, it was just a little low, less than a quart low, didn't help. I had to drive it home on the highway(about 40 mph) in low gear. I shifted thru all forword gears and it it didn't change at all, just the one low gear. I changed the filter, praying that was all it needed. NOPE!...didn't help. Any ideas? I really don't wanna spend another $900 to have it rebuilt, i just did 2 years ago....and did i mention that i don't pull or haul with thius truck. Thanks for any help, in advance...p.s. inside pan looked okay, no metal, or shavings in fluid.