XPS ribs are "probably OK" to use on your rig.
It depends upon if you cared for a more aggressive tread than that of the XPS Rib, and on whether or not Michelin uses the same sidewall ply material (not talking about the steel in the Rib sidewall plies) in the Rib that was used in the original M&S tire.
IMHO something was "wrong" with what Michelin was using in the sidewalls of the original M&S tire such that premature sidewall cracking was occuring in this tire. The new LTX M&S2 tire is using a different sidewall material than what was used in the original design and I hope that the existing XPS Rib is not using this older technology sidewall material ... irrespective of the steel in the Rib sidewalls.