the problem isn't shaving off the axle mounts and positioning new mounts higher on the axle tube, but unless someone is manufacturing a shock that can travel about 10" and is only 12-14" long total, then those upper mount (like the stock XJ mounts) will be a waste.
my lower mounts (weld-on mounts with 2 holes and a bolt-thru design) are positioned so the shock eye is about the center of the axle tube (and to the rear) and the shock body is pitched forward about 45* and the upper mount is on the inside of the ~frame-rail~ (the original stud from the lower mount welded onto a piece of plate and then stitched onto the uni-frame..
the longer the lower mounts are the better to a degree they help nuetralize axle wrap, acting as a lever in combination with the shock.
yeah the effectiveness of the shock to dampen bumps is reduced when pitched at an angle but most shocks advertise that at 45* degrees they're still 90% effective
>>and if you shave off the lower mounts and flex the rear and get under there with a tape measure you'll find that there aren't many options for mounting the upper shock eyes that'll be more effective, without going through the body, BUT in your case you don't have to worry about that, and i suppose you can always worry about that MINOR detail later and fab a new x-member..