While trying to find their way back to Rousseau in the season 1 episode Numbers, Hurley, Sayid, Charlie and Jack come upon a suspension footbridge. The "far side" posts of the bridge are still in place at the Kualoa Ranch.
The "near side" posts looks a little bit different, so I'm guessing that those shots were taken at a different location with a couple of tree parts playing the role of bridge posts.
The vertiginous cliff seen in the episode is actually just a small outcrop, at best 20 feet high, right off the dirt road that snakes its way through the ranch.
The LOST VFX team along with the actors and Sir Michael Giacchino play a huge role selling the sequence.