Why would both wheels turn when it's up off the ground in park?

When a vehicle is in park and lifted off the ground, both wheels may turn due to the differential's design. The differential allows wheels to rotate at different speeds while turning, but when the vehicle is stationary and lifted, the drive shaft can still cause both wheels to rotate together if spun. Additionally, if the transmission is engaged and the drive system is intact, turning one wheel can cause the other to turn as well due to the connection through the drivetrain.