I use a PTO driven post hole auger. It works great for planting trees from RB2s. I have heavy clay. I choose an auger diameter almost the exact size of the root ball. This allows the lateral roots to quickly penetrate the native clay. Caution: When using an auger in clay soil, depending on conditions, you can cause glazing on the sides of the hole making it difficult for roots to penetrate. An easy solution is a hand rake. It allows you to rough up the sides removing any glazing before planting.
When planting from Rootbuilder 2 containers in heavy clay, you can create a pond effect because of the water infiltration difference between the native clay and the well drained medium you use it the RB2s. The solution I found for trees that don't like wet feet was to auger the hole significantly deeper and back fill with quarry stone. This causes any pond to occur below the root ball. I also select a site where ground water would not run into the hole naturally and plant so the root ball is about an inch above the soil line. I then use native clay to from a lip down to the natural soil level. This further prevents ground water from running in and allows for some settling.