Well, here are my thoughts for locker/gear options for the 01-03 XL7.
Front: use the XL7 diff housing (cast steel prefered), load with mod'd Sami carrier, Trackick gears, EZ locker, 26 spline stock shafts, cast steel axle housing or the Anvil. Not sure the Anvil will fit though. Since I haven't had a problem with the aluminum diff housing, I don't see that as a problem as in the Trackick.
*This has already been done by Zukipilot in his article.
Rear: Keep stock axle housing. Keep the stock diff housing if the Trackick gears would fit and work. If so, configure the same as the front. For the axles, either brooch the axle splines of the locker to fit the axles or taper down the axles and respline to fit the locker case. I don't think the taper would be a problem as long as the axles are properly heat treated.
There was a guy out of CO about a year ago. Broke a Sidekick rear axle, IIRC. Had a replacement set from Spidertrax custom made. I recall Yankee Tim involved and suggestions he made. Info is located in a Sidekick/Tracker yahoo group. I can't recall which one.
By swapping in a GV/Vit rear axle, I don't see a problem with that either.