We bought a used 2004 X with the 6.0 PSD last February. Engine build date was 03/25/04. We have put 18k miles on it in the past 12 months and the only problem we have had was a leaky glowplug seal which was covered under the 100k mile warranty. The 6.0s have had problems, but there are literally 100s of thousands of them on the road. Typically you only see the negatives online.
Any used Ford diesel truck you are looking at go to
www.flatratetech.com and ask the guys there to get you the OASIS report. This will let you know what work has been done on the vehicle. Very useful info. Per our OASIS the dealer had done all the recalls and the X had been trouble free for the previous owner.
We love our X and it has been the perfect vehicle for our growing family (currently 2 kids & 2 Weims). It tows like a champ and is a pleasure to drive.