My dads 02 blazer 4x4 servicte light is going on and off it could be on flat ground hills ect..... sometimes it even knocks the lights on the push buttons out all of them even the 2wd light but when he plugs on for codes the lights out and nothing is comeing up I even pluged mine in to see if anything log in memory nothing
Im lost im gona check every thing on it fluids lines hoses conections everything just looking for a place to start maybe something common

 

it sounds like you just have a short in a 4wd sensor or your 4wd motors could be slowly dieing