if you jack up the truck look at the drive shaft the shaft from the transfer case and the front diff. and see a gease fitting on the shaft the slipjoint is right there the shaft looks like 2 peices and it is, if the slip joint is rusted it can not slide as the truck flexes, salt on the road is the killer. if it is rusted and lubing it does not help you might need to replace it. I had to replace the 1 on my wifes sportage then she traded it in on a VW Bug.

2001 5sp. K&N stock filter 215/70/15
If your check engine light is on at least you know the light still works