To add to what SGT Saur said, the HMMVW on the right is an uparmored version (most likely started as an M1025) that was done in country with a kit, not a factory M1114 Uparmored. As such, the kit has an external AC unit with it. Factory M1114s have a AC unit in the left rear area on the inside over the left rear tire and a pair of vehts along the slope of the roof. I believe the one on the left is an M1114, cant be sure though, the AC vents would be behind the Soldier's left shoulder. Reason for the AC unit, on the uparmored HMMWVs, you leave the side windows up (closed) at all times, otherwise shrapnel can still come in the window and injure or kill a crewmember. Makes the Uparmored vehicle pretty pointless if the window is still open. To keep the crew from roasting, they now have AC in them.
You can see the side vents clearly on the left rear of the below M1114. A simmilar vent is directly above this on the top of the sloped rear section.
As for the bumper, standard M998 and M1025 do not have the rear bumper, only Heavy varients (M1037, M1038, M1114, etc.) have the rear bumper. They are designed with a heavier suspension to haul/pull heavier loads. The M997 ambulance and M1114 are built on the heavy chasis to take the extra weight of the extra stuff they carry.
Hope that makes it all clear.