on my savage x I juste change my diff for the alumi one What do you mean? {shim the diff}

Shimming the diff means putting little skinny shims behind the gears to make the gears mesh better and line up properly.

-Josh-

you shouldnt really need them on a new truck. You have to be careful not to make the mesh too tight inside. Also remember to use your choice of oils.
hope this helps

thank you guys for help

New or used you need to shim them for better durability.