well, the fan does steal some power when fully engaged. But what gets me is that it is a rental with only 67k miles? I know it is not unheard of, but when a rental place spends the extra money to buy a diesel for their vehical usually it's because they want to hold it as long as possible.
Anyhow, what I would do is see if any of your local diesel shops have a good diagnostic scan (AutoEnginuity) program and check all of the important parameters (high oil pressure, injector test, etc.) to see if there is anything out of the ordinary (don't worry about #3 and #8 injector failing any tests).
Then I would start with all of the good support mods, which includes;
1) get rid of the very restrictive muffler or replace exhaust system.
2) By now your up-pipes are probably leaking. Fixing it will make a difference.
3) remove the Air intake heater which sits right infront of the turbo outlet.
4) Install a Pyrometer to make sure your new programmer/chip wont overheat your exhaust valves.
5) Add a good programmer/chip.
6) Install a regulated return or fuel crossover (riffraff FRX
) to keep #8 injector healthy and to get rid of the cackle.
These are not all required, but they sure made a big difference with my truck. If you want to know more about each mod just let us know