4x4 not engaging??????

I have a '99 f-150. aprox. 83,000 miles. my truck has the manual 4x4 lever on the floor for the transfer case, when I pull the lever to 4x4 and/or 4x4 low the lights on the dash says its in 4x4 and the front drive shaft turns, how ever my truck is not engaging in 4-wheel drive. Is there a solenoid on the front differencial? I see 2 vacuum hoses going to it. how do I test to make sure it is working? I don't think my hubs are engaging. they are automatic hubs. the vacuum lines go up to the fire wall to what appears to be 2 small soleniods. I checked all the fuses and they are good. so I think its from the pumkin or in it, or somewhere in the axle.