So, I haven't been able to engage auto 4x4 for a while. So yesterday I did some trouble shooting. I started with the pulse solenoid and hooked up my mityvac to it and got 5+ psi as soon as I clicked it to 4x4. So then I moved onto the other side of that vacuum line and was able to pressurize that side as well eliminating any vacuum leaks. Next, I jacked the front end up and moved to the drivers side and pulled the vacuum line off of the hub. It pressurized just fine when I put it into 4x4, but it was still spinning free of the front driveshaft. I heard a noise when I was pumping up the hub that sounded like the hub was trying to engage. So I pressurized it and let it unpressurize about 10 times and didn't hear the sound anymore, but the wheel was still spinning independent of the front driveshaft. When i switched it to lock it still did the same thing staying free of the driveshaft. Next, I moved over to the pax side and it did the same as the drivers side but I hear a rattle that I'm guessing might be bearings? It went away when I put it into lock. I ordered two new auto hubs with the intention that next week I will pull the outer and inner hubs off and R&R both sides. Now today, I changed the transfer case fluid and the rear diff fluid hoping for an easy win. The transfer case fluid was blackish and smelled like burnt. I replaced that and go for a drive forgetting that my truck is still in 4x4 and when I make the turn to back up it starts bucking like 4x4 is engaged. I turn 4x4 to 2wd and the bucking goes away. What I'm kind of at a loss on was, I turned both sides to "Lock" and I could spin the wheel without the transfer case moving and now it works? Any ideas? I think either way I'm taking those locking hubs off next week.

 

Is it spinning the tire on the other side?

 

The front is an open diferential.
You néed to look at the axle ujoint for the wheel you are turning. If that axle shaft is turning with wheel it's Locked in, if it's not turning with wheel its not locked in.

 

I didn't think to look at the opposite wheel. I'll get it up on Jack's again next week and let you know what I find. Thanks!