My 2009 F350 Super Duty (6.4 diesel) seems to run fine without the trailer that I've towed for two years. With the trailer behind it will barely run 35 mph with no pulling power. Check engine light is not on.

It's odd that it runs perfectly fine with no trailer. I would immediatly say you have a boost leak or turbo issue but that would still present itself as an issue without a trailer. You will need to start with scanning the truck with a good scanner to check actual values of multiple engine sensors. If you don't work on your own car then use this link to find a good shop near you. A complete hands on inspection is needed here. https://repairpal.com/repair-shops Alex