of course it would be lots easier. Use the existing axle shafts-- move left to right and vice versa, then cut out the old tube, put new in, and you're done.
I've seen people do this in their garage. "Ya just gotta want it".
Heck- I can't see it costing anywhere near $500 if you have a shop do it. It's a little grinding, a little cutting of new tube, and a little welding. I honestly think that if you're not shy of a grinder you can get out of it for less than $300.
- and another thing-- how are you planning to attach the 300 to the XJ Tranny? The bolt patterns for the two 'cases aren't the same last time I checked, and I don't believe the spline count is the same either.