Where did my front diff go?

Got to looking underneath the truck when it was on the lift at the tire store and the mechanic there pointed out that it's missing the front differential.  Now, I'm no mechanic so I didn't know what that meant so he explained that basically the truck isn't a 4x4 until I get a front diff installed.  Any idea how much getting one installed costs?  Thanks in advance and please, no ribbbing or making fun.  I already feel stupid enough.