As the guys above have said, 31 spline differential and axle setup only offers added strength. However, its not a bad idea if you can afford it. When I swapped my rear end gears from the stock 3.27's to 4.30's, I went on ahead, and upgraded to a 31 spline assembly. No sense in doing a good job twice. Not that I "need" the added strength but, it is better to be safe than sorry. If you plan on doing this, expect to spend around 500-600 dollars for a decent setup. It will work great for the street and the track. As also stated above, you can not mate a 28 spline axles or differential with a 31 spline differential or 31 spline axles. Both must match is spline number
Let us know if you have any other questions.