Slow leg

If the thrust bearing inside the leg is bad, then as the weight on it increases it will create drag which will slow the motor down. Also if the screws are worn and dry there will also be drag that will slow the motor down as the weight increases.

Just no easy answer. With any luck it will be the motor. Can you switch the slow motor out with the good one? It might take a little effort to change the wiring, but it would tell you for sure if it's the motor or not.

I'm leaning more towards an electrical rather than mechanical problem since it runs fine with no weight on it. I know what gear failure sounds like. Been there. Replaced that. I know about bent legs too. They run fine then bind, load or no load.

Maybe get a new motor then tinker with the old. If fixable I'll then have a spare that will probably never be needed.

Since 2009 the legs have been used litteraly hundreds of times so wear inside the motor should not be surprising I guess.

Good chance the brushes are dirty or the brush spring is defective. Had the same thing happen on the driver side jack. I did not catch it in time and the spring had fallen apart and the brush was stuck. If you lucky take the motor apart and clean the brushes and armature.

Is the leg bent?

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.

Take it apart and clean and oil it. Had one leg fail couple of years ago. Gear wore enough to lock up on the screw. Also check all electrical connections.