when you remove the starter and try it on the bench does it spin good? can you move the motor with a screwdriver prying on the flywheel?
when you say "The voltage at the starter is 12 Plus volts and when we crank- or try to crank the starter it drops to 11 Plus volts". where is your negative (ground) lead attached to? is there any difference if the lead is attached to the battery neg post or the engine block? where is the positive lead attached to? does it make any difference if the positive lead is attached to the battery positive post or the large connection at the starter?
if you take a jumper from battery positive to the smaller wire connection at the starter does it do the same?

when you say "All grounds are in perfect shape" how do you know? did you remove them, and clean them?
I am sorry if it sounds like I am asking very specific and very dumb questions, but I am pretty sure that what you need to find is where does the voltage drop that is failing to provide enough electric power to turn the starter