I believe that the shim keeps the back of the spider gear from rubbing on the case too. If you lose it 50-60% of the back of the gear will touch the case. That would be a huge source of drag.
Originally Posted by
BashemSmashem
It might be more significant than you think . I think its there to give the spider gears clearance from the diff case so they dont rub on the case , and since those square blocks are made of aluminum I bet the spiders gears would wear them down without the washer .
This is only my opinion , I havent had any experience running without the washers but I am sure they are there for a reason .