If the resistance, my truck, is constant for both batteries, how can the amperage ratings differ?
Because the total resistance includes the "equivalent internal resistance" of the battery. All practical (realizable) power sources have a non-zero internal resistance.
 What is does a battery's 'cold crank amperage' really mean?
"Cold Cranking Amps" means the amperage that the battery can deliver at a given cold temperature. It is specified this way because car motors are harder to start at cold temperature (they require more amperage), and lead acid batteries produce less current at cold temperatures. (This is because they are using a chemical reaction to produce current.) Thus you have a combination of the two worst cases at cold temperatures. If a battery will start a car in the middle of winter in South Dakota, you can be absolutely sure it will do great in summer in Arizona.
So, a lead-acid battery at cold temperatures APPEARS, at the terminals, to have a higher internal resistance. But it is not a higher resistance that is holding back the current flow: it is the inability of the battery to produce current at the demanded rate, because of the slower chemical reaction at lower temperatures. Manufacturers compensate by designing batteries with more reaction surface area, etc. The measure of how well they did this is reflected in the CCA rating.