Let me guess, you still have the Superuser Permissions in your app drawer, right? Try opening it and see what happens, you aren't rooted anymore, but the Rom Manager method really sux as it leaves traces of root.
I _think_ the guide linked should still work, but since you now have stock recovery, it's a crap shoot for getting it past the certs, even though the file used in that guide is an official Master FRG83G/2.2.1 file. The file will erase everything in /system, but it's the first part where it checks to see what you have now that _might_ give a problem.
All you can do is try it out.
good luck