I had ordered a new gear diff housing/plastic parts, and with those too, the gear diff was too wide. Some Dremeling means now it has about 1mm of room to move back and forth. Luckily though, thanks to the tight fit on the bearings, it stays put and moves smoothly. So either the gear diffs are always too wide, or it's indeed my gear casing that's at fault.
That's the complicated part of the story - here is the nice part. I've had contact with Durango - they responded quite quickly and stated they'd send a new gear diff and gear casing right away
Last Friday I got a note from the post office, so I went there after the weekend and the package felt quite heavy. Guess what? They've sent me a complete DEX210RTR gearbox + a NIP gear diff
I've had experience with the Durango customer service before and it's always been positive... but this is way beyond my expectations! Thanks Durango!
And no, I'm not just reacting enthousiasticly on that: The RTR gearbox moves more smoothly - the diff comes in at 19.85mm - at 0.1mm wider than a ball diff with the copper shims it is a tad wide, but that's a very reasonable tolerance within which both diffs should work nicely.