blazer 4hi

I have been having problems with the 3 button 4 wheel drive system on 1998 blazer auto trans. I replaced the actuator under the battery, replaced all vacuum lines, the vacuum switch on the transfer case and a new dash switch. I can get it to lock in 4low but not in 4 hi. when I press the 4 hi both 4hi & 4 low lights will come on then go off after a second or so. nothing happens then. I can press 4lo & it works every time. I can go back to 2 hi with no problem. when I press 4 hi you can hear the clicking from the tccm. My question is what should happen in 4 hi. I have checked all vacuum lines and the vacuum switch on the transfer case all seems ok. I do not have a vacuum when 2 hi is pressed is this normal? should 2 hi lock the front axle? any suggestions would be very helpful

I had same vehicle with alot of 4wheel issues. Dropping into 4drive without doing anything. Dropping out, flashing lights. Replaced the TCCM with a used one, never had anymore issues. Kept it for about 5yrs.