Chassis Batteries - to replace or not to replace?

[87 liberty xl]

20160830_191746.jpgHello All! I'm a new member here to post my first of probably many questions I have been following the forum for a while. I have been very impressed with the wealth of knowledge here and the willingness of the various contributors to share it. I purchased an 87 Liberty earlier this year and have been learning along the way.

I have noticed that my chassis batteries sometimes struggle to turn over the engine if it has been sitting for a while. I checked the voltage of each of the 4 batteries after letting the engine run for an hour to charge them up. With battery switch switched off, each of the batteries showed closed to 12.7 volts. When I check the next morning, 2 of the batteries are still 12.7 while the other 2 batteries are around 10.2. The 2 batteries that are doing fine seem to be much older than the two that are giving me issues.

Note: I didn't disconnect the battery cables when testing each battery. Do I need to replace the two that are losing voltage? All of them? or run a different test?

Thanks in advance for your help!

Miles