I have a 2000 Blazer LT with about every option that was available. It's got 257000+ miles on it but runs/drives great and everything still works except the 4wd. It's got the setup that has "Auto 4wd" with 4 buttons on the dash. When I push any of the 4wd options the light will flash on it for a little while and then revert back to 2hi. Any Ideas???