Locked 3.73 LS rear end

[Vic6384]

I have a 99 XLT Explorer with the 3.73 LS...

Yesterday I noticed a slight growl from the rear end when driving any speed straight, and turning gave me a much louder growl and jerkiness...
I drove to a buddy's place and he informed me that my *inside turning wheel was spinning extra, at the same rate as my outside wheel. I opened up the diff, and noted that all the gears looked fine, and the fluid was free of debris. I refilled it, hoping this would solve the problem, but to my dismay, after some figure eights with the inside tire sqeualing, it appears to have been no help.
Is there anything I can do to check the friction plates?
I'm out of ideas, and I've searched the forum, I promise...

Thanks for any help...

 

[410Fortune]

Something in the diff has gone wrong, you should have it looked at ASAP before it gets even more expensive..