If I push the button for 2 wheel it flashes then goes back to 4 low

My Blazer is stuck in 4 low. If I push the button for 2 wheel it flashes then goes back to 4 low. I have checked the relay wiring and also have disconnected the negative cables to try and reset computer. No luck. I was told that the majority of the time it is the actuator as they have a history of going out before the vacuum switch. Is there a way to tell which it is without having to spend 70 bucks for a diagnostics. Or is there possibly a fuse?

Thanks
Don