When i engage 4wd high, i hear a knock that gets more frequent as i speed up. Also 4wd low won't engage. I have an automatic tranny with electronic shifting transfer case. Transfer case is 7c34ab. Everything is normal in 2wd. Any ideas?

 

Sounds like a vacuum leak and one of your hubs might be partially engaging. Vacuum leaks are quite popular in these. Have you tried turning the hubs to "lock" on the outside and then using the transfer case to engage? See if that changes anything.

Don't mean to offend you, but how are you trying to engage 4x4 low? You're going to neutral first right?

 

No offense taken. I tried engaging 4x4 low in neutral and nothing happened so i tried rocking the truck a little and still nothing. I locked my hubs and the noise stayed the same in 4x4 high and still no noise in 2wd with hubs locked. I put the truck on a lift to try to pinpoint the noise and it sounds like it is coming from the transfer case. There wasn't any noise at all from the hubs. It did get louder when the wheels were turned though.

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Also, which vacuum lines can i check? That seems like a good possibilty since the cab and engine were recently removed.

 

On the passener side of the truck under the hood you'll see a black tank, it's the vacuum reservoir, the pump and relay are both right there, and the lines run down to the hubs from there. Just go over it carefully and check it out.

The best way to engage 4x4 low is to come to a complete stop, turn it to 4x4 high (unless you're already in 4x4 high), wait a second for the light to come on, then switch to 4x4 low. I believe your foot also needs to be on the brakes for this to work too. Once it lights up on the dash, then put it in drive again. Try that. If it goes to 4x4 low without issue and you can feel it in low range, but still have noises it's probably the hubs.

I wonder if it's possible that they pinched the vacuum line too.