To hold either a round or square post plumb inside a post hole while you pack dirt around it, make up a couple of wedges of 2x6 about 30 in. long. Cut to length, then make an angled cut from a corner of one end to the center of the other end. Insert a wedge on each side of the post; they will adjust in the hole for either post size or hole size.
While screwdrivers may look like pry bars, chisels, punches and scrapers, they should never be used for such purposes. A screwdriver is most effective when its blade is straight, balanced and sharp. Misusing yours will keep it from performing best when you need it to do what you bought it to do…drive screws. Also, be sure that the handle is clean and free of slippery oil or grease.