Hopefully they didn't fall out, Warden. They go in the cavity where the diff side seals are located.
I went thru a fiasco when I bought a differential for a swap. It turned out that was from a 300SD. It required different washers. There is a set of them available from MB and runs about $50 wholesale. My mechanic had a lot of fun fitting the right ones.
The half shaft has to have a little "slop" yet not cause the clip on the inside to be too snug in its race.
If your shims had fallen out (and you didn't notice them) the clip would most likely have been pushed off soon afterwards (and you'd have a ka-whump ka-whump noise coming off the rearend!) as the tip contacted the cluster.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!