Castle nut won't get any tighter
 

There is slight 'play' where the hub meets the axle causing the wheel to rock from side to side a few millimeters. I tried tightening the castle nut but at a certain thread it just spins...the axle and hub bolts are not where the play occurs. Do you think the castle nut just needs to be replaced? When i remove the washer the castle nut will tighten all the way but of course i need the washer present.

Sorry forgot to add - 2003 Raptor 660

Sounds more like the axle threads worn, if it will tighten on unworn threads. Try a new nut but be very careful not to overtighten (use a torque wrench). I am unfamiliar with the Raptor but would be tempted to leave the washer off, or find a thinner one, if possible, as a loose nut makes the splines wear rapidly, and you would soon need a new shaft.

thanks. the nut that was on there was not a lock nut like the other side, so its likely the previous owner lost the original and replaced it with a generic bolt that junked up over time. i could use a thinner washer but want to make sure its thick enough to handle the lateral pressure. but i think i'll pick up locking castle nut and if thats not it whatever i wasted $5...

Try 2 washers.

If you have any doubts, replace any worn parts with new. If a wheel comes off at speed, it could hurt.