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I put a little anti-seize on the washer side of the cone nut. It allows the nut to "slide" against the washer without biting into it and causing the entire axle to turn which causes your situation. Works for me ( 83,000+ miles ) but YMMV.
Believe it or not, this is a common and frustrating problem. The belt adjustment changed in '02 and, IMHO, was a step backwards. Jims even makes a tool to assist axle tightening. I had trouble with my '02 and I tied oil, antisize, grease, etc. between the nut and the cam plate and nothing worked; a new cam plate solved the problem.
There could be a couple of things going on. The cam plate could be worn and the cam bore not fitting tight to the axle allowing it to turn on the axle.
Check the flat part of the axle and make sure that the cam bore is fully engaging the flat part of the axle. The solution for this problem is to shave .060" from each spacer.
Some guys use a small "C" clamp to hold the cam plate while toquing the nut.
It is a PITA when trying to do it yourself. Check out the Jims tool
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