The axle housing and pinion mounting distances were changed in 1996 9.5" ring gear axles for enhanced durability. Currently essential GM-21777-85 Gauge Plate no longer measures the correct shim required for proper axle operation.If an incorrect shim is installed, the ring and pinion will not mesh properly, and the axle will have noise and operation complaints, or premature axle failure. GM45108 is used with GM21777-8 Rr Pilot, GM-21777-42 Frt Pilot, and GM-21777-43 Stud in place of the pinion, and GM-21777-1 Arbor & GM-21777-86 Discs in place of the differential to select the shim.