Lost 1-4 on my A4???

I drive a 2002 pontiac Trans Am ws6 with the 4l60e, here is the problem the car reverses just fine but it will not drive forward in any gears. I've tried 1st, second, third, and drive and nothing happens just revs up the RPM's. A little while ago i lost my rear end, and unfortunately i was not familiar with the difference in 3 channels and 4 channel differential rears, so that said i went out and got a used 4 channel rear end and come to realize that my car uses the 3 channel because i don't have the traction control option. well at this point i was pretty much almost done with the whole swap and figured i would try and see if it worked. At first the brakes were going crazy, so i unplugged the wire that connects on top of the rear end, the car wouldn't go no where in drive but when i shifted it into first it drove and i had to slowly shift it into 2nd, 3rd and than into drive but it would never go into drive unless i was at like 45-50mph, which made sense because the 3 channel the comp uses the reluctor ring inside the rear to determine when to shift at what speed but a 4 channel goes by the sensors at the wheels. Anyways i drove it like that for a week and than all the sudden it stopped going into any gears and just would go into reverse, but since than I've gotten a 3 channel moser 9inch and figured that would fix the problem as i thought it might of had something to do with the 4 channel and not having the reluctor ring. I've tried changing the transmission speed sensor on the tail shaft of the tranny but that didn't do the job, I've checked the fluid while the engine was running hot and in park and the fluid was pretty red and not brown looking or had the burnt smell to it, I've checked all of my fuses and none of them are blown. So point being the other thing i can try and replace would be the transmission control solenoids or just determine that the tranny was destroyed due to the use of the 4 channel rear end. Anyways sorry about writing so much but I'm hoping if someone has come across this problem or have any idea of what it could be!

You could drop the pan and see if there is any clutch residue.

Yea that's the next thing I'm gonna do is drop the pan and see whats on the inside, when i pulled out the transmission speed sensor it had like some dark slimy stuff on it and i don't think that's normal either.

the tranny problem is unto itself and had nothing to do with 3/4 channel rearend.
when you had to start off in 1st selector position,that sounds like forward sprag failure and the manual over-riding clutches allowed you to go thru the gears manually. now it appears more additional tranny damage has occurred.

agree, your problem has nothing to do with 3/4 channel rear end. seems like, at this point you need a transmission rebuild or replacement.