Here's what I've done and checked: Both keys are I rear wheels. Transmission pully spins and is not slipping. When on jacks, the wheels move when in gear but just barely (I can stop them with my hand). When in gear it is hard to push the mower, but not in neutral.
Belts appear to be OK. I took the tranny out, and spinning the pully by hand caused the axle to move without slipping, so I thought the tranny was OK. I reassembed but still no go.