I had a similar problem using MIP CVD's on a Nitro Rustler. Sometimes a good launch (wheelies sometimes did it even) would flex the suspension just enough to get the dogbone to come out. I used some of those graphite shim washers from Traxxas between the steel outdrives and the trans case to get a little better engagement. The outdrive transmission stub was just long enough to still get the setscrew to grab and hold. Also, if there isnt a flat area milled into the axle stubs out of the transmission, take a dremel or grinder to it and add a small flat so the setscrew is setting on a flat surface. They tend to come loose on a round one.

Just an idea..