The only options that we've found in the UPS and Fed Ex plugins are the 'weight approximation' fields which add a percentage to the price. We tried using the manual shipping options to create a dummy UPS and Fed Ex gate, where if it fit certain weight requirements it would use that particular plugin, but it just adds another option rather than acting as a gate to choose shipping options.
One of our coders has gotten far enough to get the proper shipping to show up now, our final hang up is that the UP option doesn't disappear once the weight goes over 75 lbs. If we can figure out how to do that, we can put the project to rest.