Why when I Check ohms from ground to both 220v legs do I get a reading of 18 to 21?

With the power off? If you are looking at transformer terminals on an unpowered unit you would be seeing the coil resistance. You should not use your meter in the ohms setting with voltage present. It wasn't designed for that. Measure voltage from ground on a 220vac transformer and get 110 vac. on each side.