Does it stay in 4-low? If so, you probably just need to adjust the linkage. If not, you may have some moisture in the vacuum system that is freezing. I had a similar problem (though no grinding) in my '91 YJ. I would put it in 4x4 after work, when it had been sitting outside all day, and it wouldn't engage at all. But when I got home and left it in the garage, where it was above freezing, it would engage right away. Gotta love vacuum disconnects!