I have a 84' Bronco with a 302 and C6 tranny. Around 60mph I get a really bad vibration throughout the truck. It is a rythmic type vibration. When I drop below 55mph it isn't that noticable. Could this be out of balance tires or something with the driveline?

84' Bronco "The Beast"

 

I'm no expert in finding vibes since I've been looking for one in my 78 Bronco for several years.:-) But here goes. A harmonic vibe is probably caused by driveline, rather than wheel out of balance. First check your driveshaft phasing in slip yoke (u-joint ends should line up with each other on both ends). Then try turning driveshaft 180 deg. I would start with driveshaft, then look at transfer case mount, engine mounts, u-joints themselves. Hope this gives you a place to start. Don

 

also check pinion angle