Your problem could be caused by either condition you mentioned. Not enough data!
You could do well by investing in a hydrometer for a few bucks(little glass device with the floating balls) and a digital volt meter(decent quality...not a junk import)
First task is to measure the specific gravity of each cell if the batteries allow access.If the batteries have reached the end of their service life,you will see one or two cells that are significantly differant than the rest(usually near the posts)This condition can cause both batteries to not charge properly.
Second task is to measure the voltage of the batteries while under charge by the alternator.To do this ,you should start the truck and let it run for 10 minutes or so. Then With the headlights on (a simple load),measure the dc voltage at the batteries. It should read 13.6 to 14.3 volts. Deviations need to be considered but ultimately this is the desired running and charging battery voltage for lead acid batteries.
Third task, but harder to accomplish ,would be to measure the current of each battery ,both charging and discharging with an ammeter "in circuit " This device is relatively expensive. An inexpensive meter could ,however, be used to measure the drain current individually of each battery. (engine OFF,key off) From the sounds of it...if your problem is caused by a short,beware that a shorted diode in the alternator can be the cause of draining batteries. (Remember this for the situation that drives you crazy trying to find the cause of a drain while off problem)
CCA's : Dont get caught up in the more cca's are better advertising. It is only relevant within manufacturers (IF the minimum CCA has been met) It does not mean that if you buy 2000 cca batteries you will double your cranking power compared to 1000 cca batteries. This only applies to the particular manufacturers test load. Buying one manufacturers higher cca could be a situation of buying crap over quality.
Do you need to buy both batteries at the same time? No..it simply helps to guestimate the remaining battery life ,if you dont know the current status. There are ways to measure the health of a battery.It is simpler to replace both, but NOT "battery blasphemy" if you decide to take the risk.