I have a 98 ranger 4wd. Today, I engaged 4wd successfully (observed lots of wind-up when I turned on dry pavement), left it engaged, and drove 11 miles. When I got to the bottom of my driveway, it had disengaged itself. The indicator on the dash still said 4wd, but the wind-up was gone. I switched it off, switched it on, and it re- engaged and I got up the driveway.
I am assuming that the hub locks let go. Something I have read says that the the locks require a vacuum pulse (a short one) to disengage, and that they will not let go without that pulse. If the hub locks did stay in, then it must be that the transfer case shifted itself out, and there is no way to tell what has happened sitting in the truck.
I have had nothing but trouble with the 4wd in the last couple of years, and in all cases it has been the hub locks. Seals have been replaced, I had a new hub, and on and on. I know that the hub locks are a real problem with the ranger. Does anybody have any advice here? The same article I read that said the locks cannot disengage by themselves also said that you can manually engage the locks if you get your finger in (remove the cover) and push on the plastic locking cams (which supposedly resemble the mechanism in a retractable ball point pen). And if you do this, they stay locked until you push again or activate the vacuum mechanism. This article said that the vacuum is NOT required to maintain the locks in whatever position they are in.