Originally Posted by imstillatwork
A np231 can be shifted between 2wd HI and 4wd HI at low speeds (30mph and under or so) without stopping.
Change between 2wd and 4wd HI under 30mph
1) clutch in
2) transfer case in 4wd HI (pull straight back, don't go into LO)
2) transfer case in 2wd (push straight forward)
3) clutch out, keep driving.
The YJ NP231 transfercase can be safely engaged into 4HI at "any legal speed
". This information was taken directly from the owner's manual and FSM.
You don't "technically" have to clutch either... Just as people with the tf999 auto don't have to go into neutral in order to achieve 4HI engagement safely. Your only going to put wear on the np231's syncro gear if your heavy on the gas/loading the drivetrain heavily while trying to engage it.
Here is a thread on the topic: Let off the gas, reach down, pull.... Thats all you need to do for 4hi. 4lo is done as you described.