You have what's called an open differential. in order to get both tires to spin you need a t-lok or a limited slip differential (LSD) you can put one of these one your stock 7.5" rear end or you can upgrade to a GT 8.8" rear end, which really isn't any cheaper or better IMO. Upgrade your rear diff and get some bigger diff. gears and you're good to go!
it's not always the same price, in some instances it's cheaper in other's it's more expensive. It all depends on the seller and the mileage on the axle. Obviously you wouldn't buy one that has a lot of mileage on it. So, in most instances, my guess would be it's cheaper to go the LSD route rather than upgrade to an 8.8 with 100k miles on it. Or maybe that's just me...If it's the same price, why wouldn't you?