2004 2500. Will go into limp mode on hwy. No lights, codes. Fixes itself. Help!

There are many things that can cause a weird, intermittent issue such as this. Your best bet is to take it to a Sprinter specialist and get it professionally diagnosed. Without codes, it could be the turbo actuator, throttle issues, air temp sensor or any number of items that require a specialist's knowledge to diagnose.