[Ol'Jim]

Downright nasty trick, I sure hope the jerks were caught, or will be. Stupid, senseless crime. The local judge ought to give them max punishment, and YEARS of community service!
Just thinking, in Minn, your temps are likely below freezing. Unless truck is in a garage, the water in the fuel lines will be frozen solid, even if you get the tank cleaned out. If the drain plug doesn't work, it might be easier to unbolt the bed and slide it back, than to drop the tank. You could remove the pump from the top and then siphon it out, (or use one of those cheapo siphon hose/pump/squeeze-bulb things, don't know how well they work). I changed the in-tank pump on my 2002 silverado about 2 months ago, and I was so grateful for the advice I got on here to remove the bed. It was really very easy. 8 bolts, I hit each with some PB Blaster, and they came right out. A couple buddies helped me lift & move the bed to the rear.
A full 26 gallon tank weighs almost 200#. And there was no risk of the thing rolling off a jack and dumping fuel everywhere. Best wishes, and keep us posted!