I have searched and found nothing conclusive about this.
My Ranger a 2004 XL 2WD 5-speed with a 2.3 has a slight vibration between 68-78 MPH. I have new rotors and wheel bearings up front, new tires and I had them check the rims for a bend. No free play in the rear wheel bearings or the u-joints. Any ideas?

 

could be 1 of 2 things

a slightly out of balance drive shaft or a separated belt in 1 of your tires

 

Could also be something upfront like ball joints, tie rod, a loose rack and pinion steering.

 

Its most likely that you're whole problem is that your tires aren't balanced perfectly. Mine has a shimmy from 75-80mph. Anything higher or lower has no vibration for me. Front end components would typically cause vibration at more speeds than just the one range.