[86k10]

You're probably getting some wear on your axle splines and side gears, that's why you have so much, but move it back and forth real slowly and you can tell where your ring and pinion backlash is, and then anything beyond that is wear on other components. That's actually quite a lot of play unless you have a genuine detroit locker, then it's normal. but if you have a normal open diff, it's just showing it's age.