2000 4x4 problem

i just got this car a few days ago i love it it runs great .i just had an issue with the 4wd button it was going into 4 lo but not into hi it keeps flashing then kicks back into 2hi but not its not even going into 4 lo anymore i called GM they told me it could be the switch on the dash . i replaced it & i still have the same problem its not going into 4wd at all not the hi or lo so if anyone knows what it could be please reply .
thanks to all of you