Are you sure that you actually measured 850 amps coming out of the batteries? That is about 10,000 watts or 13 horsepower, no starter in a 7.3 needs the equivalent of 12 hp to spin. I'm guessing, but I seem to remember reading that under normal conditions the starter draws about 300 amps to start the truck (normal, I mean not extreme cold conditions when fluids don't flow as well). If you measured 850 amp, I'd look at starter first. Oh and the GP's would add another 100 amps to the draw...a good reason to install a GPR light so you don't start the vehicle while the GPR is still drawing amperage from the batteries.