Sticky 4WD engagement - our transfer cases have a quirk where if you run it in 4wd on a surface with good traction (like a dry road or one that's just wet) the 4wd may later get balky at engaging. You can move the selector to 4 high or low, but it won't engage. The solution is to drive forwards and backwards repeatedly (on a slippery surface) until it engages. I've seen this take some time. The lesson is, make sure you run in 4wd only on slippery surfaces.