I just want to throw this out there - consider this thought experiment:
I'm not sure what model of MRAP they've got, but it's fairly large. Let's assume its model that can carry at least 6 troops. Add the operators at the front, and you have a total capacity at least 8.
There are over 2700 vehicles that were ordered. 8*2700 = 21600 men. If we consider the typical definition of a battalion, that will be 1200 men.
21600/1200 = 18. The DHS can supply mechanization to 18 battalions of troops.
I'm not sure how bad the drug war is, if that is the justification for MRAPs, but I do not understand why the National Gaurd, who is equipped already for mechanized warfare, must step aside for the DHS, which did not exist before 2001. There's very little rational reason for the expansion of the DHS.
*EDIT:
One last simple figure. If one considers a fully mobilized battallion, 1200/8 = 150 vehicles per battallion. Now, according to current US Army listings, my number (thank you, Wikipedia) for troops in a battallion is too high. Set the number to 600, and the numbers of battalions double while the numbers of vehicles per battalion are halved.
Thus you have resources for 36 battalions, each with 600 troops and access to 75 vehicles at full deployment.
Figures for estimate:
http://www.army.mil/info/organization/unitsandcommands/oud/Finally, I wasn't able to find a modern FM as of yet that lists the number of vehicles (and types of vehicles) in a modern US mech. infantry battalion (for the purposes of this comparison). Certainly, there will (or would) be other types as well if such a structure was implemented with the DHS, but the fact remains that the DHS has secured enough vehicles to fully mechanize a division OR MORE of fully armed troops in North America.