Either the BTSI solenoid like Scruf said, or the adjustment teeth in the end of the shifter cable down at the transmission. I'd start by popping the solenoid loose under the column. If you remove the knee bolster plastic and get a good look you'll see a black rod and round object (that's the solenoid) with a two wire connector to it. You can use a screwdriver, door panel tool, etc. to pry the rod's end off the shifter linkage on the column. Now see how it behaves.
If that doesn't take care of it then the adjustment teeth on the cable probably got messed up during the trans swap. I can tell you if you're not careful you can break the holy crap out of that plastic end on the cable so don't be surprised if you wind up needing to buy another; the plastic gets very brittle with age.
Be sure to use your parking brake and/or wheel chocks, don't let that sucker get away from you while you're messing with it.
Richard
