What you can see will be twice as bad from the back side. I have a few show cars and have redone many - the only way to stop it is to cut it back to good metal and go from there. It is time to make some decisions: do you care what the pickup looks like, is it time to trade it, or will it just end up being a farm beater anyway?
Is it still under rust-through warrantee? Toyota got to put a new bed on my Tundra to the time of $9000. Sure it's a farm pickup, but I wash it good to make sure it doesn't rust.