droop is much more important than upward travel.
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
great guy too.