s10 4x4 wont always engage (4 Lo)

Hi all, I have a 91 s-10 blazer 4x4, 4.3 automatic, the problem I am have is that sometimes 4 lo wont engage and some times it will. I pull the stick back while in neutral and sometimes I hear this gawd awfull grinding , and them sometimes it wont grind. Aside from adjusting the shift lever is there something else I should be looking for.? Thnks Steve