4X4 Won't lock in
 

I have a 99 F150 Flareside with the 4.2L, and the other day I went to use my 4X4 and it didn't lock in? I have been trouble shooting it a bit, and checked the vacuum lines and everything seemed ok. From the sounds of it the transfer case ingages the front axle but the hubs don't engage? Any help is greatly appreciated.