Try replacing the check valve at the end of your transfer case vent tube. Its the green vaccuum line that runs from the transfercase switch, and terminates in a check valve.

 

hmm ill have to check mine i replaced the other 2 but it acts up on hills still. is it the same kind as the other ones?

 

where are all the check valves located again? i had my ac working last summer perfect, now this summer i have wild vents and a freon leak! so i need to find the leak and replace all the check valves

 

Some trucks have two, others only have one. Usually, they are located on the passenger side of the intake manifold, possibly in the main vacuum line, before it tee's off to everywhere else, occasionally, there is also another one after the tee for the line to the switch on the t-case.

The little plastic feller on the vent line for the t-case shouldn't be a check valve.... as it needs to be able to suck in air. Putting a check valve on it shouldn't have any effect on the vents at all, unless the t-case switch is going bad.

 

New switch. replaced all 3 valves ,the one at the intake the one at the firewall was replaced with the larger valve. The size of a 35 mm film canister. And the vent tube valve was blown. I was able to suck air through it, and was able to blow air through it. Felt like a pin hole was poked in it. Drove my truck the next day 28 miles round trip, no problems.