While my 4x4 does work I've noticed when I switch from 2-Hi to 4-Hi the front end doesn't seem to engage. I have to stop, go to N and 4-Lo to get it to lock in. Not feasible if I need 4x4 on the fly going down the highway. I just replaced the vacuum line a couple of months ago in preparation for winter. Any ideas?

 

do you have a 3 button or 4 button system ?, If its a 3 button it is most likely the encoder motor , when they go bad they can lose there timing and not know where it is so it travels from one extreme to the other which is 4lo to 2hi.

 

It's a 3 button. When I push the 4-Hi I hear the solenoid 'click' and the light claims 4-Hi but the front isn't engaged. Last year I tried switching to 4-Hi on the go and I heard a slight grinding noise like they didn't want to engage. Immediately switched back to 2-Hi and stopped and did the N and 4-Lo trick.

 

I seem to remember that there is a max speed to go into 4HI...and I think it is around 40mph..?
At what speeds have you tried going into 4Hi at?

 

The 3 button NVG-233 transfer case can be selected from 2hi to 4hi at any speed.

 

Last weekend I replaced the transfer case switch thinking even if it doesn't solve the problem it probably should be replaced anyway. Now instead of nothing happening I can hear the front trying to engage but making a light grinding noise like there just isn't enough 'ummph' to get it engaged. I pulled the battery and tray and checked the diaphragm-no holes and easily pulls in by hand. Vacuum line looked good also and (thank goodness) no ATF. I'm wondering if there could be a vacuum problem based on my cruise control sometimes working-sometimes not. (I get the intermittent airbag light also.)
I'm going to pull the connector and clean it on the solenoid behind the panel on the passenger side just to be sure. I never get flashing lights on the switch-probably need to replace the actuator on the front differential.

 

Ive seen that system not quite pull the diaphragm enough because the vacuum line was disconnected at the vacuum ball in the drivers fender.

 

I wonder about that because the nipple is impossible to see on that vacuum ball.

 

My question is why does it pull in when I go to neutral and push '4-lo' and I get a satisfying 'clunk' telling me the front end is locked in?

 

That satisfying clunk is probably the transfer case going into 4 low , the front diff doesn't usually clunk when it engages . you can check if the diff is locked by jacking up the right front wheel and seeing if you can turn it when you are in one of the 4x4 modes.