Its funny you mention Armies of the Night as thats my setting...
I was thinking bout your scenario as I waited for my wife to get out of the DMV today.
What type of numbers are we talking bout here? Both yours and theirs? What type of firepower do you have? Do you have solid replaceable supplies of mortar ammo? Do you have extra food production that might be used to ply the locals towards either a revolt or just a rising to assist you?
I would think considering you have some decent artillery and maybe some armor you can take them out one at a time. Think of the attack on the 78th as it pulled out of NYC.
Hey warlord A, there is a cache of gasoline at point b 2 miles out from the city you should go get some.
In most of the situations in a T2K environment having just the support of artillery and some decent mobility/armor you should be able to tackle all but the most organized marauder groups as long as they dont overwhelm you with numbers.