Hi Bottomline, I had the same problem you had last night as I have built a new dex210 as well. I thought I had tightened the diff up tight enough, and I have built over a dozen cars over the past few years.
You can only tighten the diff so far with the diff in your hands, and you think it will be tight enough. But it is not tight enough this way. You need to have the car completely built, tires glued, and tighten the diff more, which in my case was about roughly 2 more complete turns. Then the diff nut is set more in, and driveshaft will not hit the nut anymore.
below is a picture of what it should look like.
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