A treadmill desk is really two machines bolted together: a height-adjustable desk and a motorized treadmill base. Separate them first and the move goes from impossible to manageable.
Pro tip: Photograph the desk-to-base bolt pattern and cable routing before you disconnect anything so reassembly is fast.
Watch out: Never lift a treadmill base by the deck or side rails alone; the motor housing carries most of the weight and the plastic shroud will crack.