Looking at Maxwell D. Taylor on Wiki, he seems to have been promoted to Major General of the Army of the United States on May 31, 1944 and then his rank was changed to Lieutenant Colonel of the Regular Army on June 13, 1945. He was then made Brigadier General of the Regular Army on January 24, 1948, skipping the rank of Colonel of the Regular Army entirely.

What's the deal with Army of the United States vs. Regular Army in the WWII era and do both still exist today?

Is that like a temporary field promotion?

I had never heard of that before. But a quick Google search it appears the regular army is the volunteer professional side and the army of the United States is the draft side.

Enlisted would be ranked in one or the other but officers could be ranked in both and hold different ranks. The regular army would have been more prestigious.

Army of the United States used to be only for USMA grads, or the top 10% of other Academies, i.e VMI or the Citadel.  Everyone else was RA.

Why would ring knockers be limited to leading draftees

I think you got it backwards.
When I enlisted serial numbers began with "RA."  Draftees' began with "US."
Regular army officers were few and far between and would help each other advance over the officers with "mere" reserve commissions.
When you graduated ROTC you were granted a reserve commission and could serve on active duty the entire time til retirement, but you were always at a professional disadvantage with respect to regular army officers.
Occasionally a ROTC grad would rise up to Colonel and above, but it was rare since they weren't in the "club."

Damn. Never knew that he was only a temporary Major General.

Seems kind of harsh to revert his rank back to Colonel after he led the 101st into Normandy, Holland, and Bastogne.

Not that uncommon after wars.
One reason your pension is based on highest rank held

In both WWII and Korea he was promoted very fast during the war. But 5 years after Korea he was made a General of the Regular Army. So unlike WWII, he was not demoted back to his permanent rank (He had advanced to Brigadier General between WWII and Korea.) He finished Korea as a General of the Army of the United States and about 5 years later was made General of the Regular Army.

Army of the United States