Just a fairly sinple question I think. I have a '96 Silverado z71, and whenever I go to peel out from a stop, I end up leaveing only one tire mark. Whats the deal?

Besides wearing your tire out, you're a pest to all the folks around your burn out.

As for your question, Your drive train only has enough power to break traction on one tire. Or the one tire that spins is slicker than the other. The rear differential will direct power to whichever tire will spin.
Further, most differentials are limited slip or locking. If yours is limited slip. Did someone change the diff lub without the proper additive? Perhaps your differential is failing from all your burn outs.