I bought my '99 a year ago, 4x4, automatic, and it had 106,000 on it and works great. To go into 4 LO you have to come to a complete stop, put the transmission selector into neutral THEN switch into 4 LO, other wise it will stay in 4 HI. You have to do the same thing to get back out of 4 LO, stop, go to neutral, then turn the switch. 99's do have the auto-pulse hubs that are prone to failure to lock in, but are easy to fix:either by putting a lockout kit on, or there are how-to's on here that tell about other fixes.
If the truck is overall great shape and been taken care of I wouldn't hesitate to buy it, sure beats the price of a new one ... I gave $5000 for mine last March, which was a good price for around here... I see other Rangers equipped like mine going for much more all the time, especially 4x4's.