2005 sierra sle crew cab 2wd. RPO code in the glove box states the truck is equipped with a G80 rear differential. Today I jacked the rear end up and spun one tire forward and watched the opposite tire spin backwards. How in the heck does that work? I placed this thread in lifted suspension due to the fact the truck has 10"s of lift and I'd like both wheels to drive the rear. Do I need to replace the whole diff or just buy a $1000 conversion kit? I have no use for 4wd where I live.