Help diagnose my problem!

[grayharville]

hey yall, my mom said I could have her old truck if I can fix it. Its a 1990 extended cab longbed chevy, with a GMC clip, billet grill, flowmasters, 350, auto. the motor and tranny have both been swapped in at sometime, supposedly not too long ago, we arent sure as we bought the truck used. Anyways on to the problem, the tranny is stuck in first, you can rev it as high as you like and it wont shift out of first gear, reverse works fine. What could this be? help me out please? thanks in advance!