96 chevy blazer 4wd wont engage?

the 4wd in my blazer just flashes when i pushg the on when in nutral after sevral tries 4wd low willl work but its suspose to work whn not in nutral high doesnt work at all and i dont know why this is the electric motor that engages the 4wd on the transfer case works so what could be the problem my mehcanic susgested getting my truck reflashed but i dont want to just waist the money if it wont work and as far as i know theres no way to adjust the 4wd