My scottsdale 4 wheel drive when i put it in 4 high or low it won't stay in.?

what is wrong with it ? i'll put it in 4 high and it will just keep slipping out back into 2 wheel drive. when i put it into 4 low it will pop out into neutral. it's a 1984 chevy scottsdale. it's not a complicated system i know but i am just learning about it. i put in a new engine and had the tranny rebuilt. i had a guy that works on 4 wheel drives look at my transfere case and he said someone had been in it because the chain was tight. but i think he should have rebuilt it anyway cause of it slipping out of gear like it is. is it an easy fix i can learn to do myself ?