I can find no evidence of mechanized infantry divisions in the U.S. order of battle for World War two. This is what I expected. Armoured personnel carriers were new and expensive throughout World War Two, and for all nations their use was generally limited to the (third) infantry regiment of armoured divisions, and to the reconnaissance and heavy weapons support platoons and companies of other divisions.
The U.S. Army officially designated just five divisions as motorized infantry in World War Two, and only for the period 1942-3 (The 5th was a phantom, part of USFAG for the D-Day deception):
However, I am unable to determine the extent to which these units were truly motorized. Typically they would have been assigned to Armoured Corps, where their extra mobility would be most useful.
There were also three light division, designated respectively as Alpine (the 10th), (mule-)Pack (the 71st), and Truck (the 89th). The last appears to have been a fully motorized division, but deployed to Le Havre only on 21 January, 1945 and saw only relatively light action. It did participate in the overrun of Buchenwald on 4 April 1945.
The remainder of U.S. Army infantry divisions, excepting mountain and airborne, were foot-transported. However, note that the mobility of a foot-transported divisions is more commonly limited by its logistics than by walking pace; in this all U.S. Army divisions were supplied with plentiful trucks for transport of headquarters units, artillery, supplies, and the rest of the tail.
Where the German Army relied on horse-drawn logistical support for the entire war, most likely due to the extreme scarcity of valuable gasoline and diesel fuel, the Allied armies were much more mobile due to being able to relocate support troops, supplies and headquarters units forward much more quickly. This can be seen in the speed of the Allied advance across France after Avranches and Falais.
In closing, recall the vast number of landing craft that the Allies, and particularly the U.S., employed throughout World War Two in both major theatres. These are, in essence, floating Armoured Personnel Carriers. The use of precious production capacity in their manufacture instead of simpler tracked APC's seems, to me, eminently reasonable.