If you have auto hubs, stop the truck, engage neutral, push in the clutch, and shift the transfer case from 2h to 4h, then if you wish to 4lo. Drive gently until you feel the front hubs clunk into engagement, the "lock" light on the dash lights up, and you're in 4wd. Use only on dirt or slippery pavement. To disengage the hubs, stop the truck, engage neutral and push in the clutch and shift the transfer case back to 2h. Then drive the truck gently in the opposite direction to the direction you were last moving in 4wd about 12-15'. A careful ear can hear the hubs disengage, and the "lock" light on the dash will go out.

If you have manual hubs (have "lock" and "free" on the dial on the wheel hub center), get out of the truck and turn the dials to "lock". Shift into 4h or 4lo. Ditto on dirt or icy asphalt. Shift the t-case out of 4wd and unlock the hubs to return to 2wd.

Max speeds in gears in low range are about half the max speeds in hi range, because the lorange gear reduction is around 1.9:1. If you give me your tire size, I can compute your speed in each gear at redline and 3000 to give you some guidelines, but the tach still works in 4wd....