4x4 problem

Hello,

I have a 1999 Ford Ranger Xlt 3.0 V6. My 4x4 will not engage and I can't figure out why. It turn the *** to 4x4 high and the light says that it is in, but the front wont lock in. The front drive shaft does spin and if I try to spin it while it is in 4x4 I cant budge it. So i jacked up the front end and both tires will spin without spinning the front drive shaft. Is this a problem with my hubs, or is there a vacuum system on this that I should check. I looked at the lines on both tires but they look good. Also is there a pump that I should check or anything you could think of?

Thanks in advance,
Tyler