As others have suggested the uni's maybe close to their limit; I'd also suggest the overlap of the PTO shaft should be checked to determine if it's sufficent at that operating angle so as not to bow, induce excess vibration, or disconnect/split in two halves when engaged/torque is applied (can't comment on the Speeco PHD but some manufacturers have minimal overlap of the PTO sections in the expectation of a near vertical auger operation).
Our preference is for using post drivers where practical, but regularly drill holes mechanically at acute angles,using either PHD as pictured (depending on how many holes you're digging you may wish to upgrade to something similar):
- A (red) Proline PHD - a mechanical PTO PHD where the angles are only limited by the top link/linkage adjustment as the PTO shaft remains virtually straight at all times transfering drive through a chain to the elevated gearbox (yes, you can add as many drill/auger extension rods through the gearbox to go as deep as you want asthe tractor/PHD will power)
- A (silver) Digga Hydraulic PHD - the red ram in the frame provides downforce, a ram mounted in the frame connects to the drive box & alters/controls the auger motor drive angle & combined with a hyd top link/linkage adjustment you can quickly set a hole at any angle you want & being hydraulic driven the auger can be reversed (the drive box also disconnects & mounts on a FEL in a few minutes)