You have a serious grounding problem,if you are grounded at all.
As mentioned about underground utilities. If you have UG wiring the grounding rod could have pierced a hot lead. This would cause voltage to ground and neutral with the main off.
When the condensing unit was changed and a new disconnect installed,did they also install a ground rod?
Have you disconnected the main,then read voltage,then start turning off breakers to see when or if the voltage goes away?