1990 Ranger no Overdrive after trans swap

1990 Ford Ranger, 4.0L, 4WD, 165K on body, 123K on engine, 100K on tranny.

I recently bought a 1990 Ranger Super Cab with a bad auto tranny and swaped in the auto tranny from my 1990 Ranger Long Bed (bad frame). After changing drive shafts (they're about 2 inches different), the new(er) Ranger won't go into overdrive. Shortly after, it wouldn't shift automatically between any gears, I had to pull the shift lever into 1st, then put it in 2nd, then 3rd or it would stay 1st until it got up to operating temp. My mechanic found the modulator was bad and leaking fluid into the engine throught the vacuum line. He replaced that, and now it shifts fine from 1st to 3rd, but not into OD. I can let off the gas at 60 MPH (engine turning 3200 RPM) and it will drop to ~1000 RPM, but will not go into OD. I had no problems with the tranny in the old truck to the day we swapped the tranny. While the tranny was being swapped, we flushed it and changed the filter, although the fluid wasn't nasty or burnt. Another mechanic said that the computers might be different and I should try swaping them, another said the truck was too old to have a computer contolled tranny and that the kick down cable must be the problem. Any ideas or help would be greatly appreciated!