Also, the terms are used a bit interchangeably but for our trucks low pinion is standard rotation and high pinion is reverse rotation.
Just to add to what I said, low pinions front and rear will give you a strong rear and weak front going forward and a weak rear and strong front going backward.
Low pinion rear and high pinion front will give you two strong diffs going forward and two weak diffs going backward.
High pinions front and rear will give you a weak rear and strong front going forward and a strong rear and weak front going backward.