when to change trans fluid 2000 blazer

 hi new one here .my son inlaw says that i should'nt change my transmission fluid that i can add when needed never heard of this .and when i push the all wheel drive button it just blinks and if i put it in low 4 wheel drive it goes in but i have to play with it keep pushing the bottons back to 2 wheel hi its a 2000 4 door 4 wheel drive blazer4.1 vortec.any advise thanks

yea u should change ur trans. fluid. about every 50k miles depending on how hard u drive the vehicle. sounds like either ur 4x4 switch is goin bad or the encoder motor is goin bad.