EGR and P0171?

Ok, I've got a 1st gen with the dreaded P0171 (system too lean) code. I checked for vacuum leaks and found none, then cleaned the MAF sensor 3 times and finally the code didnt return. I'm thinking im a smart son of gun until i opened the hood up later on and realized id left a vacuum hose off when i cleaned the MAF sensor. This is the hose that runs from the air box to the EGR (exhaust gas recirculation) box. After reconnecting the line, the code came back. Any ideas? I know this system only works at high rpms or heavy load so with the truck idling there's no vacuum at all. How do you check it to see if its working properly? Could it be another vacuum leak i just havent found or and oxygen sensor?