I called the dealer and found out that while they have a selection of spacer thicknesses they did not stock one thin enough to fit my GSP's.
I measured the gap between the inner diff housing and the snap ring slot using a feeler gauge with the axle pushed all the way into the diff.

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily