I had the chance to pick up a Venezuelan export XJ from a surplus sale at their consulate in Germany. It had a front 44, but my guess is they are really rare.
I'm sure somebody makes a swap from a later model vehicle (TJ maybe?). But is it really cost effective? Most guys beef up their D 30.
Trying to make a bullet proof XJ is often a lifelong project and many times the next piece in the drivetrain fails (U-joints etc.) after you install one heavy duty component. Th D 30 isn't necessarily the weakest link, besides an occasion set of carrier or pinion bearings (after 150,000 miles or so), I've never had much trouble out of one.