Most will have an orange sticker in the door jam telling you. If not you can lift up both rear tires at the same time. Turn one tire by hand, if the opposite side goes the same direction, it has a LSD. If opposite tire goes opposite way, it is an open diff. Be careful when lifting the truck, the transmission has to be in Neutral to do this. DOnt run over yourself.

I just want to tell you both good luck, and we're all counting on you.