Kind of hard to tell if it's electrical or mechanical. And not that easy to check. It could be the thrust bearing going out and causing more load as the weight on the leg increases. Or it could be the motor going out. Or it could be the gearbox is going out. I don't know of an easy way to tell without taking everything apart.
About the only thing you can easily check, if you have the equipment, is to see if both motors pull the same amps when not under load. If the slow one is drawing more, that would point to the motor or gearbox being bad.