There are some TSBs (TSB-03-13-001, TSB-04-13-011, TSB-05-13-009, TSB-07-13-002) for this code. It involves a PCM reflash, and on '01 models it may also involve replacing all four of the oxygen sensors. If this hasn't been performed on your truck already, the dealership should have told you about it. If it has been performed, the first thing I'd check would be the oxygen sensors, assuming you don't have any exhaust or vacuum leaks. I've never fixed one with this DTC before, but I looked on Mitchell and on iATN and this is my best guess.

'91 LWB, 31x10.5s, KYBs, A/T "I look like Robert De Niro, I drive a Mitsubishi Pajero" - Billy Bragg (not the original lyrics, but that's how he sang it when I saw him in Dublin in '04)