just bought a 2001 reg cab flatbed cummins and put in in 4 wheel drive yesterday and the front tires werent spinning..?? nothing wrong with the t-case cause i rolled underneith and the front drive shaft wont spin freely once i engage it. what do i look for and check??

 

This happens all the time because these model trucks have air driven lockers check for a black hose that goes to your front axle make sure it has a good seal also check where it comes from.

 

It's CAD (central axle disconnect) large pressed hunk of metal beside your diff, you cant miss it. It connect and disconnect the axleshaft via vacuum. first check your vacuum line make sure it's not disconnected.