Sorry if these are stupid question. On my truck the vacuum diaphram pulls the cable. What else happens? Does the TCCM do something that allows the front wheels to lock in? Is the fork being pulled out on the transfer case turned or just pulled towards the passenger wheel? I've found a couple of transfer cases in a junkyard. Exactly what am I looking for to make sure the ratio is the same?

 

Lets first get the terminology correct. The transfer case is what is bolted to the back of the transmission. It is shifted either via a manual lever on the floor or an electronic actuator on the side of the transfer case.

When the transfer case shifts into 4HI or 4LO, the vacuum switch is activated sending engine manifold vacuum to the front axle engagement actuator. The actuator pulls on the engagement cable which, in turn, pulls on the shift fork inside the front axle. The shift fork engages a collar over the differential stub shaft and the passenger side extension axle.

Now, to find the axle ratio, you must compare the RPO codes of your truck with that of the donor. Search for 'axle+gear+rpo' and you should find the relevant details. The search feature works!!