Never rebuilt one of these transmissions (manual) but I have experience with a rock crusher 4 spd. The popping out of gear may be a worn shift fork. Have you looked at:
http://www.drivetrain.com/parts_catalog/manual_transmission_overhaul_kits/suzuki_sidekickand__geo_trackers_overhaul_kit.htmlfor a rebuild kit? The manual transmissions in the 2nd generation trucks are referred to as the "M59" tranny for Trackers. I would only guess the Suzuki GV tranny is very similar if not the same (simply because most parts within my Chevy Trackers have the Suzuki stamp or mold on them). If you only had stiff shifting issues I would recommend you try a different gear oil in your tranny case like Royal Purple or something, but different fluid probably won't improve a worn fork. Although I have NOT rebuilt one of these, I would be game to try WITH a good FSM and a really smart guy I could call for questions. Separating the cases without knicking or chipping them and the small coil springs and detente balls would probably be the biggest pain to navigate through.
Finding one in a junk yard (around here) is next to impossible. Even then it would have high miles and you don't know what you are getting. GM Parts will sell me a new one for around $2800 so attempting to rebuild one with newer synchronizers is appealing.
http://www.gmpartsdirect.com/catalog/frameset.cfmI have no idea what a "tranny shop" would charge to rebuild one but I'm of the mind buying a new one would be "more better" than this option because of the labor charges they would have to charge and the fact all the parts would be new: all bearings, seals, forks, shafts, etc.