"Free" as in the 'free' position or as in their working operation?
If anything, have you looked at the rear shaft...joints? Damage of any kind? T-case oil, output shaft?
Free as in the free position yes, rear shaft & ujoints look/feel good, tcase oil not sure and output shaft haven't checked, wouldn't either of those cause vibrations possibly at varying speeds not just 60-70mph? Literally when I hit 60 I start to feel the whole truck shake & I've never gone faster than 70 so I would assume it would on keep shaking past 70...
