My 68 J2000 has the rear shackles flipped down. I've had no issues with vibrations, binding, or flipping back at all. Been that way since I bought it, 9 years ago. I don't see the problem.
Zack - 68 J2000, AMC 327, 4bbl intake, dual exhaust, Pertronix upgrade, Holley 600cfm, T18, dana 20 (twin sticked), 3" body lift w/ 35x12.50 MTRs ~ running AND driving!