I have just purchased a 1998 F-150 Lariat Ext. Cab with manual shift four wheel drive on the floor........I recently had the gasket replaced on the front differential now when I shift to four wheel drive the transfer case will shift, but no power to front wheels......Help!

 

the front axle is still locked in by the GEM module via a vaccum signal. My guess is that they either knocked off the vac line or gummed up the works in some way. look on the axle and be sure that the lines are on the motor on top of the axle.