I had some similar thoughts when I was working on the history of Maxwell's Militia (a Morrow Project Encounter Group). This is the solution I developed to explain the origin of a feudal system with tanks
Artillery solves many of your problems. Self-propelled units are best, but towed pieces will also work.
1. Each tank, artillery piece, or mortar (or even something like the Mk19 grenade launcher) is assigned to an appropriately sized farming community.
2. The determination of "appropriate sized" depends on the range of the weapon. It should be able to hit anything within its area of responsibility. A little overlap is also desirable
3. All farming communities should have some form of (small) multi-story fortification. This could be built from scratch or be the result of modifying an existing building. They would probably look something like the PELE or PEEL towers of the Scottish Border region. Or maybe a Martello Tower. These towers would be a lookout,a place of refuge for local women/children and also serve as a command/control/communications position for the Artillery
http://en.wikipedia.org/wiki/Pele_tower
http://en.wikipedia.org/wiki/Martello_tower
4. Farmers, villagers, etcetera should be given light weaponry (shotguns and hunting rifles are OK), appropriate training and some way of communicating with the artillery - for directing fire against marauders
5. As well as Artillery, the military should maintain some form of highly mobile "Quick Reaction Force" that will respond to any incursion. Tanks, APCs and SP Artillery are great for this role but horse cavalry might be used.