According to the manuals, not over 55 mph. Anything under that should be okay, assuming the transfer case is in good working order. I don't like switching at speeds much above 40. There really shouldn't ever be a need to though. That being said, I found my Nitro will shift from 2 wheel drive to 4 hi at 75 mph. I wouldn't recommend doing it though.
01 TJ - 4.0L, NV3550, 2" Skyjacker budget boost, Xenon flat flares, Trail Gear front bumper, Tuff Stuff rear bumper, RH4X4 sliders, 33" STT Pros on 15x8s