Umm I don't know what if any of this post has to do with DARWIN

The OP is asking what American Units served in the Pacific arena during WW2.
After looking myself I decided not to try and post all of the ARMY and MARINE Divisions which are made up of Brigades and Battalions. In effect they had more in some of their divisions than we had in the Pacific War. While most of our troops were elsewhere fighting Germany and Italy. The vast majority of our Pacific contingent were made up of Reservist's drawn into the conflict by the Japanese when they threatened invasion of Australia.
From: Lilpurplepearl.
I doubt we would've been eating a lot of rice before the war was over, but I won't get started on that.I don't know what your knowledge is of the Pacific theatre war but Australia could have been made the Base for the Japanese should they have decided. WE didn't have enough troops to defend ourselves in early 1942 and only their preoccupation with annoying the USA into dropping their embargo on them, which backfired so badly and ended in America joining the war. This stopped the invasion and took Japanese troops away from Australia and into the Indo-Pacific Islands which then created the full blown Pacific war.
Should Japan have decided to use its full fleet, its full Air force and It's full army on a preemptive simultanous strike on the main three eastern Australian Capitals. We would have had a very different war.
I wonder if Winston and the European Generals would have let the Australians return to try and get the Japanese out of Australia.

Pig Iron Bob wanted to draw a line at Brisbane and not let the Japanese go any further south

I wonder who told him they would start at Rocky or somewhere north.

Neil