Fuel filter is the cheapest, easiest, and most common thing so I'd start there.
It is very similar but his didn't require a towing load to set it off and it always threw some codes. In fact, if it's not throwing codes it's probably not in limp mode because limp mode is caused by a code and will turn on the check engine light. So without a check engine light you have a problem with something that the computer isn't monitoring (maybe low air flow, turbo/intake leak, or bad fuel). I'd check those filters, drain the water separator in the fuel filter (though I think there's a sensor for that), and see if you can get a computer that will monitor codes and sensors while driving (such as autoenginuity or similar).
Sorry I don't have a quick answer for you but maybe someone else does.