You could probably remove the pump without dropping the pan, but for a good installation you should remove the oil pan - the lower pump area provides the flange seal area for the pan. Getting the pan off with the engine in the truck may be a bit of a challenge. To completely remove the pan I think the engine has to be jacked a little. I haven't tried it with the engine in the truck, though - so maybe someone else can be of more help.

Note the picture below - you'll need to remove the oil pickup screen/tube which is bolted to the pump housing - so you might as well do whatever's necessary to get the pan out of the way:


Frank