If your XJ has ABS, then it came with the D35. Could be any number of reasons that the PO swapped out the axles.
As far as using an 8.25 or an 8.8? The 8.25 is a plenty strong axle for most wheeling, it will handle up to 35's with no issues, and is bolt in. The 8.8 is a good option, too, but...