Please help!! 1988 GMC Truck and oil leak

Our 1998 GMC 4 wheel drive truck has sprung a leak from the oil filter. My husband changed it twice but both times once we start the engine the oil just pours out of the filter. We noticed it moved (the filter) slightly when the truck starts. We are sure we have the correct filter. 1.) What could be causing this? Someone told us it could be the oil pump that's cause this to happen. 2) Is it true that the engine has to be pulled out to replace the pump on this truck?

THANKS

Make sure the gasket from th old filter is not stuck on the housing.

The gasket is a common problem but if the filter is blowing out I would suspect the pressure relief valve is not working.
On older GM engines it is in the side of the housing where you attach the filter. The dealer will sell you a new one and a new gasket or o-ring. You can also purchase a used one from a scrap yard.
You will notice a split filter housing if this is occuring.