I know this is an old thread but I just came across it. I had also heard from numerous sources that radials were a bad idea with swing axles. There was even a Mercedes newsletter cautioning that radial tires make the "breakaway" of the rear worse, and to take curves slower.
Years ago I was driving my 1966 Mercedes 250S W108. It had the newer hydro-pnuematic compensator on the axle rather than the older spring, and I was running radials. I never had an issue before, but... One day I was getting off a parkway exit with a decreasing radius turn. Halfway through I realized I had taken it a bit too fast and instinctively let up on the gas. Bang! The car did a 180 and I ended up in the grass.
If I ever own another swing axle Mercedes, which I hope to some day again, it will have bias ply tires for sure!