A sticker should be stuck to the sun visor on the driver side that
tells you the rules of the system.  If that's not there, it should be
just like my 89.  4wd use only in slippery conditions. (no center
differential in the transfer case)  You can shift into 4WD High at any
time though you may need to let off the gas to allow the shift to
occur.  you can shift out of 4WD high at any time, though it may not
come out immediately if tension is on the driveline.  i.e. it may come
out of 4wd a little way down the road.

4WD low you must be stopped and in neutral to engage and disengage.
you may need to back up for a few feet after you disengage 4WD low to
get it to release.

http://www.therangerstation.com may have more info for you on your
truck.