If you read through this thread you would know it's not called a posi trac. But enough on that.
The only thing on an 8 and 3/4 on the end of the axles that would resemble a shim is the metal gasket between the housing and the backing plate. That is not a shim that is a gasket or seal.
However if you were to post a picture maybe we would better understand what you're referring to as a shim.
The original 8 and 3/4 has an adjuster on the right axle so you can adjust the end play in your differential.
That's if you're using original bearings. If you are using what they call the green bearings then end play is basically adjusted by grinding the end of the axle off or removing the Cross Pin from the differential.