8.8 swap

The Explorer axles with discs previous to 98' are 31 spline as well, it's the Ranger 8.8's with drums that are 28 spline. I'd have to go back and research to see when 31 spline started, but I do know that, for example, the 97' Explorer axles are 31 spline.

The weight difference comes from the fact that the 8 3/4 axles are banjo axles. Banjo style axles are an older design, though I'm not sure if that is good for bad. The carrier is accessed from the front. Some would argue that the heavier 8.8 is stronger.