IMO, there are a few major reasons...

When you lift the vehicle you increase the vertical distance between the planes of the front output of the t-case and the axle pinion. Anything that raises the pinion decreases that distance and is a good thing. The less angle you have the longer joints last - single or double. Also, the less angle the more abuse (torque) the joint will take before failing. Bigger/stickier tires 35"+, gearing - both t-case/axle, turbo/superchargers, higher displacement engine swaps, t-case swaps/doubler box install - consider all of these before making your axle choice because they all will increase the stress on your drive line - especially the u-joints. If you can see ANY of these things in the future it's better to build your axles correctly NOW than have to redo everything later.

Note, some of these combination absolutely require a HP and/or exclude the ability to run at road speeds with the front axle engaged, or have some other (one or more) irritating drawback(s).

Depending on how you're planning on driving with the front axle engaged also makes a difference. The HP lubricates differently than the LP. The HP, IMHO, works better when the pinion is run at extreme angles. If you're planning on using the front on the highway (up around 50 MPH) then the less angle there is the better the diff will oil itself and the more likely it will (if set up correctly) run vibration free.

This is VERY important if you're planning on running deep gears at road speeds: 5.xx+:1. Even with full synthetic gear oil there is a LOT of heat generated in a differential running at road speeds. If there isn't adequate oiling then you will do EXACTLY what I did - weld the pinion to the housing. This isn't as much of an issue on the trail where you're bouncing around and sloshing oil everywhere and the speeds are VERY low - but, of course that depends on trail conditions. If it's 105-110F degrees when you're wheeling all the time, then....

Getting the drive shaft to run without vibrating at road speed - if this is a concern - is important. At those speeds, especially if driven frequently, you will beat the crud out of your pinion bearing, t-case output shaft bearings, u-joints and drive shaft slip joint in short order.

Drive shaft protection is usually very important. Anything that raises the drive shaft at the pinion makes it less likely the DS will get hit or you'll land on it. Lots of big rocks on tight trails and shelves will make a hash of a low riding DS, t-case and pinion bearing quickly.

Not all front Dana 44s are the same, either. If I remember correctly the axle tubes on the HP are 1/2" thick. The LP are 1/4" or 3/8" - something like that, anyway they're the same thickness as the tubes on the Dana 44 rear axle. Again, if I'm not mistaken, that includes that XJ axle.

As some of the guys have pointed out it's possible in some cases to use a LP. I just depends on what you're doing with your vehicle. The best advice I can give is to think ahead when building your vehicle. It'll save lots of cash and headaches down the road if you do that. In most cases you probably can use either a HP or LP but there are some cases where either a HP will last longer, or in some way decrease costs (short term or long term) or you have no choice and you MUST use a HP.

Edward