I am thinking of replacing the differential on my 1983 240d. I drive the car on the highway quite alot. At 60 miles per hour I feel that the rpm's are just too high on the old engine. I have read most of the post about replacing the differential. What I want to know is, what do the shims do, and will I need to have extras on hand before the swap? I figure that the shims take up space so that the axle doesn't "klunk" when the axel swings through its arc. Anyone with experience either changing axels or differentials please help me out. Thanks, Dave

I believe the shims do take up the 'free" space between the axles and the diff and they vary from axle to axle.
When I replaced the axles on my 82, the factory spacers did not fit and I ended up putting the original leftside spacer on the right axle and then surface ground the right spacer to fit the left axle.
I used a feeler gauge to measure the gap between the axle end and the diff in order to know how thin to grind the spacer.

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1981 240D 143k 4 spd manual -SOLD

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

Yes, the shims are specific to axle/differential combinations. Changing either MAY require different shim thickness.