The Jeep manual says that you can "shift on-the-fly while moving at any legal speed" between 2HI and 4HI...
I have shifted from 2HI to 4HI at around 45-50 MPH in both my Jeeps before without any problems.... I'm not saying this is necessarily good thing to do, but when your on a slippery highway you have no choice, and it will engauge...
However, you CAN NOT shift into 4LO unless you are going SLOWER than 2-3 MPH, or you probably will snap crackle and pop in the transfer-case :mrgreen:
-Nick :!: