You can drive at highway speeds in 4x4 high range, but I don't think that you really want to go much faster than 55. If your in 4x4 high on the highway, there must be a reason for it - like the roads are really crappy, with about a foot of fresh snow, and your pulling a 4 place snowmobile trailer loaded down. You can shift in and out of 4 high at any speed while you are moving. To go into 4x4 low range, you really should be stopped, and top speed would be about 35mph.
There is no reason to have 4x4 engaged on dry pavement, and could cause wear on the system.
By the way, it's a Jeep, not a car!
89' YJ - 4.2 - NV4500 - NP231w/sye - 1" Body Lift - 2 1/2" Rancho Suspension Lift - 31" Firestone MTR - AR Outlaw II - Tomken Bumpers F & R - Warn 8000 front - Warn 6000 rec. mount rear - FlowFit Seats - Tom Woods Rear & Custom 2" front driveshafts