back end vs rearward

back end and rearward both are nouns.

Word	Noun	Adjective	Verb	Adverb
back end	Yes	No	No	No
rearward	Yes	Yes	No	Yes

As nouns, rearward is a hyponym of back end; that is, rearward is a word with a more specific, narrower meaning than back end:

Other hyponyms of back end include nape, nucha, scruff.

back end (noun)	rearward (noun)
the side of an object that is opposite its front	direction toward the rear

back end (adjective)	rearward (adjective)
	located in or toward the back or rear
	directed or moving toward the rear

back end (adverb)	rearward (adverb)
	at or to or toward the back or rear