You mention that you're such a perfectionist when building the gearboxes, yet later on you mention that there is exessive play in the mesh
To me this is not building them to perfection.
No need to spend hours building gearboxes. I had my Rango for about a year and half, and never had problems with the diff. Same with the R that I had too. I just biult them, had a quick check for exessive play, and if all felt fine then it went on track, with no problems racing nats and regionals for well over a year.
I was going to mention bearing collapse but Mr Skelding beat me to it!
Also, with both of the durangos I had, when you build the diff case, the 3 screws in the casings have all been facing down. Maybe this is why your car went backwards?