Chevrolet Repair: No power in overdrive, leaky seals, chevy c1500

Question
89 chevy c1500 305 5.0 L original motor and trans both been rebuilt..trans has 36,000 on rebuild..first and second gears ok, reverse ok..when it shifts from second ( automatic) into what is supposed to be drive or overdrive, which gear shifter is in, it has no power revs up high then drops back into second

Answer
HI TODD,THIS COULD BE FROMA COUPLE THINGS, FIRST IS THE SHIFT SOLENOID OR SHIFT VALVE IN THE VALVE BODY IS STICKING NOT ALLOWING IT TO GO INTO THE NEXT HIGHER GEAR, OR THE TRANSMISSION IS LOOSING INTERNAL PRESSURES,FROM LEAKY SEALS OR THE OVERDRIVE HIGH GEAR CLUTCHES ARE BAD, BECAUSE THEY WERE SLIPPING ALL ALONG AND YOU DID NOT KNOW THIS BECAUSE IT WAS SO SLIGHT. THIS COULD OF HAD A SEAL FAILURE WHEN IT WAS REBUILT..I WILL ASSUME YOU HAVE CHECKED THE FLUID ??? I THOUGHT SO..CK CHECKING. I THINK THE BEST THING IS TO TAKE IT BACK TO THE PLACE THAT REBUILT IT AND SEE WAHT THEY SAY AND GET BACK TO ME IF YOU WANT TO... GOOD LUCK TO YOU,,TOM.....