My city public works trucks are both 2008 Fords with the 6.4.  Both were purchased new and both have around 70k miles on them.  Earlier this year the F550 developed a low power output issue and threw codes for a low fuel pressure problem.  A friend of mine is a Ford mechanic and recommended I replace the fuel filters 2 times but bet it was an injector pump.  I replaced the filter and it ran OK for a bit but did it again.  We took it to a shop and they replaced the injector pump at the cost of $4,500.