Re: 1998 UP Explorer - not engaging 4WD
The only electronic part here is the TCCM, or transfer case control module. If it was bad you wouldn't be able to select low range, it would flash at you and go back to your previous drive mode.
First off, check the transfer case fluid level. If it's fine, then I'd suspect a broken fork. The fork is what the TCCM slides back/forward to engage/disengage 4wd. Which would explain why 4LO still works. If your transfer case has only the one fork to push/pull the gear to lock it into 4wd, then it could be a bearing issue or one of the shafts has worn significantly enough to slip the front axle assembly altogether.
Now, the easiest way to check yourself is to find a friend with a hoist, or several GOOD jackstands and get all four wheels off the ground, and with rpm below 1000, and 4WD engaged, see if the front axle turns (just bend over, don't go under....). Who knows, the front differential might even be the problem....
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!