Most likely a C Clip axle and some in and out play is normal. If excessive over .040 + - and you either have a worn button on the axle or a combination of a worn C Clip and cross shaft or axle button or posi clutch stack. If it is a non C clip axle you need a new wheel bearing. But some in and out play on a C clip axle is normal. Measure it and post back. Hope this helps. Most likely an 8.8 Posi. (just assuming) and if the clutch packs are worn the axle play increases. It has to be checked if excessive.
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"As I lay rubber down the street, I pray for traction I can keep, but if I spin and begin to slide, please dear God protect my ride"
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1989 Mustang GT right axle moves in and out 
