Rear shimmy at 50+mph

First make sure the valve extenders aren't leaking - which can always be an unseen problem on the inner wheels. If the wheels didn't cause shimmy before playing around with the extensions they shouldn't shimmy now. What kind of extenders are you talking about?....the solid metal ones which are actually valves and installed at a tire dealer; or the rubber ones or the braided ones that screw onto the original valves??.....these are the ones that tend to leak if not installed very carefully. Lots of postings about air pressure problems with the braided screw on extensions. Not everyone has those problems, but lots do.