Either of these threads will point you in the right direction:
4x4 No Worky.... Maybe this will help....
4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO
Don't let the title of the 2nd one throw you off. It has some good info about the engagement of the front axle.

Threads copied from the Tech Article (DIY) index.

 

pulled the battery and battery tray so that i could use some vice grips to check to make sure the 4x4 works and this is what i found....

 

That would explain a few things. While you are in there, lubricate the cable and work it to get the lubrication all the way down the cable. This will keep things working properly for quite awhile after you get that issue fixed!

 

do you think i should just buy another actuator or should i convert to the manual cable like one guy shows how to do on here?

i guess what i am really after is... how long should this actuator last before i will be fixing it again?

 

What you are asking for is purely preferential. I don't think I would do the cable mod. If I were to do anything to change things (ie try and make it better), I would try to make it somehow be electric rather than vacuum operated, but I just like to tinker. LOL

 

new actuator installed and still not working!!! i am about to give up.

 

pulled off the vacuum line running into the actuator and i have vacuum even in 2wd. so i see that it could be a bad mechanical switch on the transfer case. but if it has vacuum all the time why wouldnt it engage into 4wd when i press the button? if it always has vacuum then the 4wd should be working all the time. right?

the only other thing i think it could be would be a broken cable from the actuator down.

 

It may not be strong vacuum if there is a problem with the switch.

 

well i took out the two bolts that hold the actuator bracket to the fender and put the battery tray back in with the battery so i could watch the actuator while it was working....


as soon as the engine starts it creates vacuum and pulls the actuator and cable completely. i disconnected the hose from the actuator and put my thumb over the hose and it has enough vacuum to almost leave a hickey on my thumb.


its like the cable being pulled isnt really doing anything...

i am hoping its something simple like the cable being broke.

i am going to try to disconnect the cable at the bottom and start it up to see if the other end of it is moving or not. i know i am going to need the vacuum switch though because i have vacuum regardless of it being in 2hi or 4hi or 4lo. already found that part at advanced auto for 15 bucks so no biggie. if its the cable i plan on getting one off a junkyard vehicle.

but lets say the cable is working correctly... does that mean it might be the return spring you discuss in the thread talking about the front axle clicking? how hard is that to change out?

i mean i have swapped motors in cars before and pulled trannies and such but dang i really dont wanna get into all that.

oh well i guess this is what to expect of a truck i got for 1300 bucks.

 

How far is the cable being pulled. It needs to go at least a 1/2" but 3/4" should be sure to engage the front axle.

The 2nd link I posted quite a ways above has pictures of what's inside the front diff if you have to go there.