I may be way off base here happens all the time . . .
Start the Beast up, with foot on brake shift to 'N'. What does the 'wheel indicator lights' do? How about the '4Lo' light, what is that doing? (on, off, or flashing)
If the front tire lights are flashing, then turn the 4WD selector back to '2WD'. Now shift into a forward or reverse gear, move the truck a few feet. Now select the opposite direction. Place the shifter back in Park. The front tire lights should now be off. Now turn the selector to '4Hi' and put the Beast back in gear. Does it go into 4WD now?
I know yer thinking you know how to put the Beast in 4WD. But I'm thinking that you don't do it too often. If you accidently attempted to put it in 4Lo without being in 'N' things wont work correctly. Similarly, if you had the wheel turned or were on good traction, it won't always respond immediatly even trying to put it in 4Hi.
Let us know what you have and what you tried.