My 4x4 indicator light is on even though I am in 2H / truck had been sitting for a few months 1998 F150 v8 triton

 

sounds like the actutor on the tcase isnt properly engaging try looking under the truck at the end of the stick where the pivot arm is and see if all the bolts on the bracket are in good working order sometimes they manage to snap or come out and that can allow the bracket to turn slightly and thus when you shift ranges the bracket may move slightly and the connecting rod may not completely shift the range, and the indicator is seeing that it isnt completely shifted.

 

this happens to my truck also....but only wen its well below freezing....things that have fixed it are simply letting the truck fully warm up or backing up...now mine may b different because i have the manual 4 shifter