In theory, as long as you are going perfectly straight, without undulations on the road, then 4 Hi as fast as you want.

If you have to do any turning and the tires cannot slip, even just a little, then you get drivetrain wind-up.

It's a part-time 4wd system designed primarily as a RWD platform, which is why the basic schematic goes:

Transmission -> Input Shaft -> Planetary Gear (for when you engage 4-Lo) -> Rear Drive Shaft

When you engage 4wd Hi, you are now engaging the front wheels via chain (because gear driven t-cases are loud) at the same rate as the rear wheels.

If you start turning, then the front and rear axles turn at a different rate and you get this if you it for an extended period at high traction:

http://rubicon-trail.com/4WD101/driveline-bind.html

 

Without a center differential, I assume the Jeep would act kinda funny in an avoidance maneuver while in 4wd. Maybe something to try in a big empty parking lot? Just don't flip it on the lid