Jerry, are you saying that the sun gears are what wear? The planet gears, or both?
There is an area of the sun gear that engages the carrier assembly directly for high range, disabling the planetary system. This small toothed area can wear tapered, causing it to overpower the shift motor and disengage as a heavy loads is applied. Sometimes the carrier also wears, and both parts must be replaced. That's when the checkbook tends to have a coronary!
My Motor magazine heavy truck manual from the mid-1970's has a very good section on rear ends, including 2-speed service and overhaul. I've found a pretty good selection of those manuals, and also original factory service manuals on Ebay, usually at pretty reasonable prices.
Jerry
