In addition to what Tom has mentioned,
What kind of wheels/tires, lift do you have?
Could one of your wheels be slightly "out of round"?
Or one of your tires have a blown belt?
Is your front end slightly out of alignment?
It's kind of difficult to diagnose when we don't know what kind of equipment you have fitted to your Liberty...
For example, our KJ also has a nasty vibration through the steering wheel between 70 and 85 KPH (43.5 to 53 MPH).
Above that speed range it drives absolutely smooth, but it is due to the fact that I need new tires (Drivers side severely cupped and just worn down) and an alignment.
BTW, do you have any photo's of your Rig?
I'd like to see your lift that you have fitted.
Cheers