carjack vs highjack

carjack is not a noun while highjack is a noun.

Word	Noun	Adjective	Verb	Adverb
carjack	No	No	Yes	No
highjack	Yes	No	Yes	No

As verbs, highjack is a hypernym of carjack; that is, highjack is a word with a broader meaning than carjack:

carjack: take someone's car from him by force, usually with the intention of stealing it
highjack: take arbitrarily or by force

Other hypernyms of carjack include commandeer, hijack, pirate.

carjack (noun)	highjack (noun)
	seizure of a vehicle in transit either to rob it or divert it to an alternate destination

carjack (verb)	highjack (verb)
take someone's car from him by force, usually with the intention of stealing it	take arbitrarily or by force