Why won't my jeepS four wheel drive system "shift on the fly"?

[Juevos]

My 1990 jeep Cherokee pioneer oftentimes will not shift on the fly. I know it can't go into 4wl while moving but 4wh won't work either. When I try to pull the lever it fights back and sometime sounds like there's is grinding. Randomly I can finally get it into 4wd but rarely. Also, sometimes when it is engaged it just jolt violently back and forth every 5 feet or so. But it still works just fine occasionally. Help please