He has a 2001 1500
It will have rear wheel ABS at a minimum.
Additional question is whether he also has FRONT ABS. Take a look at the brake master cylinder and look for how many lines. Or look at a front wheel and see if there is an additional line to it (1 of them being from the brake fluid).
I wonder if you got the correct sensor for the rear differential.
Will you elaborate as to what triggered this altogether? What made you replace the tranny sensor? That one has a specific code for it.