I have a '94 4-banger with 4" lift and 31" tires, and I can do 70mph, so you should not have any problems doing 70. I would not recommend doing 80mph in a Jeep, not only because of safety (the jeep will hit 80mph, bt if you have to make an emergency manouver, you might find yourself in a bind) but also because of mpg (Jeeps have the aerodynamic characteristics of a large brick, so the faster you go, the less mpg you get, especially after 50mph or so).
Felipe