The low pinion (standard rear and solid axle front) diffs are weak when used in the front when the rig is going forward because the way the gears are cut.  Gears have a drive and coast side.  The drive side is much stronger so it works great in the back.  When you put a low pinion in the front it is on the coast side of the gears which is by many estimates at least 20% weaker as a front vs as a rear.

The high pinion is much stronger when going forward than the low pinion for just the opposite reasons as the low pinion.  The only time a HP is weakish is going backwards which runs the gears on the coast side.