Acting up above 50 mph

I just got this truck the other day and it's still under
warranty but I'm just curious if I can fix this without
using the warranty. When I test drove it it ran great
no shaking or pulling one direction or anything.
Been driving it the last few days with no issues at
all. Today I was driving on the interstate and it
started acting like it was hydroplaning but the
farther down the road we went the worse it got and
the check engine light started flashing. The truck
shakes and doesn't have acceleration on even a
slight hill. It pretty much just holds it's speed if I put
the pedal to the floor. When I do that it downshifts
and the shaking seems to be less than driving in
the final drive gear. If I'm driving through town or
less than 50 you can't tell anything is wrong and it
just has a steady engine light unless I stop then it
goes away. What is wrong with this??