'99 Regal GS New Tranny won't downshift

I've got a 99 Buick regal gs, that I just replaced the tranny in, from what I've read, it looks like I need to have the shifting patterns reset (TAPS).

First of all what is the easiest way to determine which tranny I have.

Second, my symptoms are that erratically (more bad than good) the tranny will not shift down at a stop. When I pull the manual shift lever back it will only go into 2nd, and when I move it up it will only go into 3rd. no first, no OD.

I have read that there are several sensors, and things that could be going bad. I took it to the tranny shop and the guy thought that it was prolly the computer. Me I don't think thats the case. I have access to several interchangeable computers, but they have to be programmed before they work. (and I'd rather not pay for that at the dealership if it's really not what it is) Is it possible to DIY change the automatice transmission Input shaft speed sensor, Vechile speed sensor, or check modulator line?

ps the new tranny reportedly has 47k miles on it. (the original tranny was stolen before I bought the car)

looking at the reciept from the tranny shop I had 8 codes from while I was repairing it (it would be nice to know what codes I'm getting now)
1860 TCC PWM Solenoid Circuit Electrical
713 Transmission Fluid Temperature (TFT) Sensor Circuit High Input
717 Input Speed Sensor Circuit Low Input
748 Pressure Control Solenoid Circuit Electrical
753 1-2 Shift Solenoid Circuit Electrical
758 2-3 Shift Solenoid Circuit Electrical
1602 Knock Sensor (KS) Module Performance
1810 TFP Valve Position Switch Circuit

He said that the reason it was shifting weird was because it was getting low voltage. He thought that it was because the computer was bad, but I really don't think that's what it is.

Erratic-check your modulator.

Can you be more specific.

I think my transmission is electronicly controlled.
Where is this modulator??? How do I check it?
What is the sensor that the shifter linkage hooks to called? I don't know if when I put the new(used) transmission in if that was with that transmission, or what...would that make a differnece?

Well I went back to the transmission shop today to talk to the mechanic again, and he told me that the only problem with my transmission is that when it goes on the fritz (which is most of the time) that the tranny is actually loosing power. He said that since it was outside of the tranny it was outside of his territory.

Any one have any suggestions?