2000 Blazer 4x4 not working

I have the push button 4x4 and can't get it to engage in 4x4 Auto. I took it to a shop and they said that it will shift into 4x4 in 4Hi and 4Lo, but when they try it in 4x4 Auto I just get the Service 4WD light on my dash and the 4x4 Auto light just blinks. You can here clicking from the module on the firewall then it just faults. They said that it does have vacuum and changed the actuator. That didn't fix the problem. He said that he is getting voltage at the solenoid that goes to the actuator, but it's not sending vacuum to it. The solenoid is new and you can hear it clicking. They said that I have two positive voltages going to the solenoid and should only have one and a negative. They suspect that I lost ground going to it. What they did in the mean time was put the vacuum lines going to the solenoid and actuator together. This is giving me a constant vacuum and allows the 4x4 to work. Will this harm anything? They think that the PCM might be bad. Does this sound right? Any help would be great. Thanks in advance.