To expound on the topic, the NP231 is a chain-driven transfer case that really isn't designed for high-speed operation. It just plain isn't good for the chain. It is OK to shift into 4H at any safe speed. The xfer case will NOT shift into 4L unless you are stopped or nearly stopped.
Also, the NP231 is not an all-wheel-drive xfer case, but an on-demand case. There is no differential in the xfer case. So, if you're on dry pavement and you turn while moving, you get a difference in rotation between front and rear driveshafts, and due to tire traction, the driveline gets bound up. This is the fastest way to snap U-joints, axle shafts, steering knuckles/joints, or the xfer case itself.
Off road, you get wheel slippage, so driveline binding doesn't occurr as severely.