Re: 98 Navigator 4x4 problem
I hate to say it, but it sound like age. Older 4x4 vehicles, with a manual, or electronic 4wd selector generally get to the point where you have to drive in reverse to change it from 4HI to 2HI.
Otherwise, you'll have two things to consider. One, if you have the knob (electronically controlled transfer case), the TCCM (transfer case control module) could be starting to fail. It won't always throw an engine code when it fails, it'll just be a PITA for you when you want to use 4wd.
Second, the forks in the transfer case that move the gear from 4HI to 2HI are damaged, or worn. If you're lucky, replacing the transfer case fluid (every 30-40K and I recommend synthetic) may help with the shifting some, but it won't cure whatever the problem is.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!