You'll need a limited slip kit unless you want to change the entire rear end.. It'll consist of axle side gears & a few bits & pieces to lock them together when you lose traction on one wheel. There are also "air lockers" available that are really the cats meow, but they are very expensive. Google "Randys Ring & Pinion" they get high marks for the lowest prices & they ship when they say they will.
If you cant afford a locker right away next time you spin a wheel try putting the "E" brake on, not hard enough to stop the truck just enough to get power to transfer to the other wheel where hopefully you have at least a little traction.
If neither wheel can get traction then even a locker would've helped you.
Lockers are trickey to use in icey conditions, if it was just snow theres no real problem but when encountering ice a locker tends to make the truck very squirrelly especially when turning. This is the main reason why I always recommend using a selectable locker, when it isnt wanted or would cause problems it can be left off.
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