Think of it as also pushing the front and rear axle down. It lifts both the body and frame, and thus improves approach, break over, and departure angles. The height of the diffs and their ground clearance remains unchanged, so clearance under the diffs is not improved with spring spacers alone, but the other benefits are significant.
Larger tires and/or portal axles will raise the diffs and improve clearance of the axles in addition to the body and frame. A 33" tire will fit easily on a stock G-class. A taller spring or spring spacer will help for even larger tires.
I run a 33" tire with 30mm spring spacers. Looks, rides, and drives great.
Do you guys run them on stock springs? I'm need new front shocks on my 1991 w463 SWB, thinking of getting bilstein, but keeping stock springs and adding theses spring spacers. (will end up on 31s" I believe)
id get the spring spacers and not a body lift if you were doing just tires.
when you wheel you try to keep the rock in line with the tires, and the body lift helps with break over and approach/depart. it also allows welding on sliders and keeping breakover the same.
a large spring only lift is often an expensive proposition to tune out correctly, and a combo spring/body helps to get the body up without sacrificing much in drivability, and you dont have to spend bucks on correcting the castor angle caused by a large spring lift. in fact, in trucks like my fj60, the combo works best. it kinda depends on the raw platform design stock.
i notice you have a 300gd. i dont think i'd do a thing. it has 88hp. i wouldnt want to ad tire diameter or weight to 88hp. enjoy it as is, then move "up" to something in the 200+ tq range before adding bigger tires.
The only thing that increases clearance under the axles is bigger tires. There are many ways to make bigger tires fit, and one of them is spacers. They aren't the best choice because of issues like articulation, but when coupled with bigger tires, they will get you more clearance.
due to body design, many spring lifts require lowering the bump stop.
the most you'll do on a G with a spring spacer or increased spring height, without having to deal with the axle is 2 inches (50-60mm).
so, the answer is, "it depends" and a spacer may be not only the best option, but the cheapest.
ive lifted and wheeled and overlanded with fords, a bunch of LC's and a gwagon. that being said, unless i had some compelling reason for 35's on a G i wouldnt do it, and for 33's you only need a 50mm spring packer and stock springs for full articulation on a pre mbusa 463. mine is a '99, and i cant say for certain the axle width on a 91' but if its narrower, it'll be that much harder. i'd only do springs if i had to swap them because they were beat. mine have 300000 miles on them, so id second guess the necessity offhand.
that reminds me. i think a 91 will have bearing issues if you over tire it. i seem to recall the mexigermans site suggesting so. most of what you need is there. 4x4abc.com