Having used a power auger on more than one occasion to sink holes in the
yard (once so as to sink concrete pier-columns for a deck; plus on three
other miscellaneous 4x4 post-related occasions) I have learned that for
every two holes attempted, one hole will have at least one huge rock in the
way requiring excavation and removal by hand-shovel; a very labor intensive
and time consuming process for just one (out of shape) person such as
myself.

Find a company that rents tents for large functions, and see
if you can get them to drive your fence-posts with their
pnuematic tent-stake drivers? Those things hit hard enough
to mushroom the end of #12 rebar.