In my YJ, whenever I'm driving in 4wd high and go to hit the brakes a terrible grinding noise happens most of the time. Similar sound to when you miss a shift. It doesn't matter if the car is in neutral when I brake or in gear the noise sometimes happens. The grinding noise doesn't exist in 2wd, just in 4wd. I'm assuming transfer case but I want to know for sure

 

This question is probably better suited for JeepForum.com or maybe even WranglerForum.com, most guys here are answering the usual JK related questions and not many YJ folks hanging around. In On tother two forums they've still got a good presence of older model owners who may be able to confirm what may be a common issue with that particular vehicle.

Best of luck with the fix, hopefully it's something easy like u-joints in the front axle.