I'm for the ground being bad as posted earlier. Makes more sense.
Hydrostatic lock while running could only occur if you poured water into the intake, or filled the oil pan with a bad fuel pump (civilian gas engine style w/ bad diaphram)
For "both" trucks to do this, with one doing it 'after parked' likely indicates you have a bad connection on the frame/starter/battery. Friends deuce did the same thing when he came out of a gas station, I towed him home, then diagnosed it as the starter connection...started right up.
The PS manual states you should turn the engine over with the fuel shut off pulled out for 5 seconds! (see tech section and read) then push the fuel cut off in and crank for no more than 30 seconds. This is after you have PMCS'd checking the engine oil before starting in the morning...why...Hydrostatic Lock from fuel leaking into the cylinder.
BUT it never mentioned anything about driving along and locking up. Now the one that was running and died, may have other issues, as failure of the electrical system should not kill the engine once started unless it also lost suction for fuel, as the fuel pump only needs to pump during starting (normally) then the Injector pump can pull the fuel up even with a bad fuel pump or the accessory switch in the off position. (try it, rotate your switch to off without pulling the fuel cutoff out...still runs, just no juice...)
The thought of a locked up starter due to mud packing the housing is another possibility on them, but it should not have killed it running, and the one that drove around should not have merely frozen up overnight. Were mud packing the case, I'd try inserting a pressure washer or an adapted steel brakeline to a garden hose stuck up into the bell housing hole before I pulled the whole transmission (ugh!) the steel line should get in enough to clean it out, possibly rotating the transmission once or twice to get at the crud in the clutch. (much easier than pulling it...)
PLEASE post the results so we can learn...
V/R
LTC Winget
US Infantry
Kandahar, Afghanistan (for 6 More days!!!)
Virginia Military Preservation Association
www.vmpa.us