There are swivel speaker mounts (used some at our church) but I don't recall the brand -- a Google search should turn up some at many places (Best Buy, etc.) Double-check the weight rating. You might also look for a TV swivel wall mount that you can use, or modify to use, with your JBLs.
HTH - Don
p.s. You could also screw some bolts/eye hooks into the speaker and matching hooks on the wall, using them and some cable/rope/ties/bungees/whatever to suspend the speakers. The vibration may mess up the imaging a bit if they are not tied down well, but you'll probably not notice on surrounds.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley