A sentence is supposed to stand alone grammatically.
Especially if you are dehydrated.
That is a fragment, pure and simple.
I'm not even convinced that @Tcovo is correct.
Especially dehydrated hikers should stay away from foods with a high salt content.
This sentence is grammatically correct, but could be misread. Technically, the sentence refers to "hikers who are unusually dehydrated." If you meant to say something closer to "Everyone should avoid salt, especially hikers who happen to be dehydrated" then you are on the wrong track.
Especially large trucks must stay in the right lane.
Again, this sentence is grammatically correct. It refers to trucks that are unusually large. It could also be written "Trucks that are especially large must stay in the right lane." However, the sentence doesn't convey much useful information. What is an "especially large truck"? I'm not a truck driver but I'm pretty sure a truck driver wouldn't know what makes a truck "especially large" either. But it's grammatical.