Have your alignment checked, more specifically your cambre. I know someone that had an explorer and it would start shimmying at like 50 and stop around 60 and after months of getting different things fixed he finally took it to his friends garage (he races d sports) and his cambre was off.
__________________ 1995 GT 5.0
CAI, American Thunder Cat-back, Mac O/R H-pipe, 180* thermostat, Steeda Tri-Ax shifter, Steeda rear swaybar, Urethane bushings, MSD coil, King Cobra clutch & flywheel, GT40 Intake Manifold, ~1800 watt sound system | 
Shimmy n' Shake 
