Yukon A-body axles are actually 27.875" long. This does not provide enough spline engagement for early clutch sure-grips with 2 piece side gears. You need axles that are made to the correct length, or a NEW sure-grip with 1 piece side gears.

BTW, I stock new clutch sure-grips, correct length axles, powder-coated, bolt-in A-body 8 3/4" housings, drum and disc brakes and just about anything else you can think of for 8 3/4" and Dana 60 rears.