Recently at my cousin's place I saw some Bobcat fencing attachments which included 2 augers, one was about 4" in diameter while the other was a bit bigger (6 - 8"?) and they were at least 36" long.
The link shows a picture of a Bobcat like the one he uses to pull his post pounder around on the job and he can drill a pilot hole with the auger if necessary. It's a pretty sweet lil machine but it cost him a few shekels. He can carry a lot of tools, wire and stuff, including his different sized augers in the back of the Bobcat. Unhook the material bucket and substitute with any attachment you want. I guess when you are in it full time like he is, you can justify the expense. He has the one pictured in the top right corner.
http://www.bobcat.com/attachments/auger
What I used to do if the pounding was tough was use my old single prong, heavy square tube bale spear to punch a hole in the ground. Just tip it straight down and poke it into into the ground. Between the weight of the tractor and wiggling it slightly forward and backward, I could make a 36" deep pilot hole into some pretty tough soils in the summer time.
I can't do it anymore since I cut it off and converted it into a double prong spear for big squares. I don't miss it one bit though, because it was about the most useless thing I ever saw for moving big round bales, being too bulky to push into bales easily.