On my 01blazer zr2 ls when I push the auto or 4hi the lights keep flashing. When I do it when I'm stopped I here something engaging but light keeps flashing. I've checked the actuator under the battery and looked fine no rips, vacuum lines look good and so does the vent. Is it the switch on my dash or the vacuum switch on the transfer . Help!

 

Sometimes it works right away sometimes does not

 

Possibly a vacuum leak elsewhere. If there's any vacuum leak it will affect lots of things like HVAC controls. The lines may have fluid in them, so they "look" ok from the outside but internally they're full of fluid which isn't allowing the transfer case to engage.
Try searching around in here because there are lots of posts with similiar issues. They include proper ways of testing the electrical etc.

 

Thanks maybe the actuator is corroded and doesn't work sometimes. Also the light some times doesn't even light up if I hit the dash sometimes will come back on . How do you get the fluid out of the lines if there is some in it?

 

Found out it was the solenoid on the firewall not letting vacuum through . But does anyone know how to change the u joints on the front drive shaft? I see one at the diff but the one at the back feels like it has play but how do I change that one?