Per a post I saw somewhere:

"Axle Data
Gen 1
2.6, 2.3TD, 2.5TD & 2.5TDI (some) :
front axle = 25 spline, 29mm diameter, diff = 7.25" diameter (low pinion)
rear axle = 28 spline, 30.5mm diameter, diff = 8" diameter
3.0V6 :
front axle = 28 spline, 29mm diameter, diff = 8" diameter (low pinion)
rear axle = 28 spline, 30.5mm diameter, diff = 9" diameter

Gen 2

3.0V6 :
same as Gen 1 except for NH & NJ auto has 7.25" diameter front diff (high pinion) NH & NJ 5sp man has 8" diameter front diff (high pinion)
2.5TDI :
same as NH & NJ auto
3.5V6 & 2.8TDI :-
front axle = 28 spline, 29mm diameter, diff = 8" diameter (high pinion)
rear axle = 31 spline, 33.5mm diameter, diff = 9.5" diameter"

Do high and low pinion refer to taller or shorter pinion gears on the ring gear assembly or the input pinion? Are these what affect whether the 2.85 or 3.15 gears can be used?

1991 Montero LS 3.0 5-Spd LWB 31" Bridgestone Dueller ATs on stock rims, tan/tan interior/exterior
1986 Isuzu Trooper 2.3 5-spd - partially disassembled, tan interior/dark blue exterior