I recently went mudding in my 71 Blazer and noticed at one point when I was stuck that one of my rear tires wasn't spinning as fast as the other one. I know that I have a posi rear end because I have jacked up the rear and both tires spin in the same direction. What would cause this? Is it just worn out? Do I need to add something other than gear oil? I haven't pulled the cover yet so I have no clue what is in their.