Transfer case not shifting into 4 wheel in front passenger tire makes

There is a vacuum operated diaphragm on the passenger side of the front axle which initiates a fork to engage axle in 4 wheel drive. This diaphragm sometimes will not work because of vacuum source, fork damaged, diaphragm damage or lubrication sticks fork. This is where I would look first, to isolate this as potential problem. Please make sure all tires are at proper pressure and sizing first as even one worn tire will mimic this problem on older higher mileage vehicles .