Yes, reserving a specific vehicle is available sometimes. For instance, I often see the MB GL450 available at a few larger/southern locations, when booking on <24 hours notice. However, other than the MB, I haven't seen any specific large SUVs available for a specific model. Some sedans/cool cars are that way, but very few and far between as they don't know what will be on the lot (accidents/late returns/recalls/maintenance/etc cause cars unexpectedly to be out of the fleet).