Ok, while that may have seemed like a strange question, it was getting to something, which apparently isn't the issue.
Here is the first thing to do.
Remove the batteries and run them down to the autoparts store to have them load tested. Yes, even if they are only 6 months old. Replace both batteries if either one tests marginally or fail.
Second, place the batteries on a battery charger. Ensure they have 100% charge for the test you will do next. Then reinstall your batteries.
What those two steps do is to eliminate bad battery connections and marginal batteries giving you a bad reading for the FICM. Batteries are cheaper than a FICM and you would rather replace those than it.
The last step in this process is to actually test the FICM. Here are specific written directions on how to do this. TESTING the FICM
Truck has to be cold, batteries charged to get the best possible results.
The directions in that link will tell you what to look for.