Hmm, well, I think I had to remove the FICM test from the links in the signature line to make room for a worthy cause.
for a good write up on how to do it.
Listen to what was mentioned before about your batteries. Low voltage from the batteries can fry the FICM, and if you test the FICM with bad batteries you will not get a good result. That may cause you to purchase a FICM when your underlying issue is a bad battery, or your new FICM can get fried from your old batteries.
As previously mentioned, take the batteries out of the truck and run them down to your local auto parts store. Ask them to load test them for you. It is a free test. If either test marginal or fails, replace as a set.
When you get the batteries home, charge them up to a 100% charge. Even if you bought new batteries, charge them since they do not come off the shelf fully charged.
Before installing the batteries check the cables over and check/clean the connectors. After doing all of the above, then you should check your FICM voltage.
For between $195 to $325 (depending on what length warranty you want), you can have your FICM repaired. ficmrepair.com, there are also other places that sell upgraded FICM's that raise the voltage output to 58 volts (stock is 48 volts). There are also places that can install a "performance flash" on the FICM as well. You have a ton of options.