Don't panic. It's not that bad. Don't drive the truck. Drain the tank and if you can get to the fuel filter then drain that also. Get a diesel fuel treatment and put in tank with fresh diesel. You may have to crank it a bit to get it started but once it starts let it run without driving it until you hear the engine smooth out. I've seen it happen a lot and it's much worse if you put diesel into a gas vehicle. This whole process should tak you less than an hour and the truck should be ok.