I've looked at this more and there is no spacer behind the slipper, its just the layshaft spacer that wears. I have had this apart many times, my spacer is good and the slipper is thoroughly assembled according to the instructions and it still slips a lot. I've burned three sets of new slipper pads in about a minute. Its not a complicated device.
Any ideas? I am ready to scrap the stock slipper and go with an avid. Its hard to believe that Durango makes a slipper that doesn't work like this and everyone running the truck has gone aftermarket.