If you do in fact have a 28 spline rear axle, don't worry about it for general all around street use. If set up properly, you'll really have to try and tear it up. 28 spline axles have a scallop that takes up almost the whole axle end that you can see when the wheel is removed. 31's have 3 holes on the end-the middle one being about 1/4" diameter, and to it's side the other 2 are about 3/16". About in there somewhere.