Check all of the vacuum lines. If they're original, it's past time for replacement.
3....are you talking about the vacuum switch on top of the case? If you used a Dorman...take it out and get the updated GM switch. I believe it has a blue stripe. @oldeerslayer can tell you for sure. Make sure you replace the lines to the switch one at a time.

 

test vacuum
if not at least 15 inches at idle youve got problems.
If you have a hand held vacuum pump,try that and see if the actuator is strong enough.
Otherwise it may be an internal problem in the differential

 

Get about 12-15' of 5/32" vacuum line and just replace all the hoses from the engine to the vacuum ball, T at the firewall back to the transfer case switch, switch to the right front fender actuator one at a time. Maybe a new vacuum switch at the transfer case to in the new design. I just did all this on my 02' Sonoma crew cab and everything is working fine again.

 

As Rhotpursuit said if you used a Dorman brand TC vacuum switch they don't fit right and leak which will cause the actuator to not function properly. Granted they're $20 cheaper, but if they don't work it's wasted money. You have the right idea to check the vacuum at the actuator end of the hose and as stated by dime-dozen you should have around 15 inches at the end of the hose. The other possibility, if you have adequate vacuum, is the cable is binding. Shoot lots of PB Blaster down the end and move it back and forth. They're not too bad a job to replace.