I know what you're going through. I just moved into a house wih a nice stand of trees and paths in the back. Have gotten "poison whatever" three times in the past six months.
I recently cut some vines off at the base of an old Sycamore tree that were probably 100 feet high and 6-8 inches in diameter. I thought it was just Kudzu (sp?) or some other nasty vine trying to choke the tree. 36 hours later and I've got rashes again.
Though I can't offer any solutions, I figure misery loves company. Hopefully someone with some expertise in this area will read our posts and offer a solution. If I could identify all the stuff, I guess I'd put on a rubber suit and eradicate all I can, then take the suit to Yucca mountain.
I gotta get that stuff out before my kids start getting rashes, and I won't give up on those trees!