You're good on your PCB and GPC if your starter solenoid is clicking, so you have other problems. This is probably a stupid question, but are you sure you've hooked your batteries up in series, and that those batteries are charged? Sounds like it's trying to run on 12V instead of 24. There are diagrams on the web, but basically, the cable coming off the block with fins mounted on the side of the battery box goes to the negative [-] of Battery 1. The cable coming off the stud going through the battery box wall towards the front of the battery box goes to the positive [+] on Battery 2. Then jumper the remaining two terminals to one-another (one will be +, the other -, and obviously on different batteries.)
You also could have the Infamous Ground Problem, so check all your connections. Go underneath and check the physical connections to the starter and make sure both ground and battery + are good and clean. Also make sure the starter bolts are in their tight, and that the fore-end mounting nut is there and tight (sometimes that falls off, or is left off by lazy mechs, and the starter will torque under load and sort of jam.)
Go under the hood and run a jumper ground (set of decent jumper cables works) between a nice, paintless piece of engine metal and one of the studs/ bolts that mounts the PCB. After you've done all that, try start it again.
There is lots and lots of information on Steelsoldiers already concerning this and other common HMMWV problems. Check the 'Stickies' first. I've had starter problems before which were basically just tired starters, and/or solenoids with contacts that were worn out 15 yrs ago.
