coming off the top of my head, the axle spline count changed in the 8.5 rears in trucks in '88, and in i believe '91 in suburbans and blazers.. when they went from the old school square to the early 90's body style.. the '88 up OBS and NBS axle spline counts are the same.. in '99 the 10 bolt rear end became the 8.6 from the 8.5.. it utilizes larger bearings on the carrier.. the ring and pinions are all the same for 8.5 and 8.6.. like mentioned above, you can use different bearings to make the carriers interchangeable..

not hi-jacking, just gonna throw out a lil bit of info.. i hope its correct..