AWD problem (discouraged)

Hi

I have had a problem with my 4wd for about 2 years. It started when I changed my encoder motor to a brand new one (dorman). My rear tires began to spin when I was turning (left or right). My mechanic told me that it showed that it was working now. and not before. There was something that wouldn't disengage and my differential was forcing. Last week, I changed the whole transfer case (TNT transfer case rebuilt), I had the rear differential oil changed (everything is fine, nothing broken inside), I had the discs changed , pads and caliper on the 4 wheels as well as the pedal break pads. I also changed the 2 CV joints. All brand new. Unfortunately, randomly, it may still have the same problem! Sometimes, it went back to doing the same thing. I turn left or right and he steers. I press on the accelerator pedal and I feel strain. The rear wheels spin. I do not know what to do. Most of the time, it works correctly, but with 4500$ spent, i want my problem to be fixed for good. Could it be a sensor abs problem? We read a code on the right front wheel yesterday.

thanks a lot.