What you describe sugests an electrical malfunction but certainly not what or where. It would require precision placement of resistance in order to duplicate what is happening. At this point I suggest taking the motor to an electric motor shop for testing. If you know your way around a multi-meter, you might test the supplies before changing the motor. Above all,be carful. I'm supprised you havn't been shocked or worse already.

I would start with the relay in the indoor unit first. The thermostat is my second choice. There are other factors, such as a motor overheating, but your description doesn't reflect that.

The "delta T" is referred to as the split temperature - the difference in temp between the air before the a/c coil and after the a/c coil. Your split temps are good. The slower air speed will provide better dehumidification and cooler temps coming from the a/c coil. You can get the manual for the air handler from Lennox (or look at the wiring diagram inside the air handler) to find out what speed your fan is set on (low, med, med-high, high). I believe that the "24" in "10ACB24-9P" means that it is a 2-ton unit (24,000 btu). I think that a rule of thumb is 300 cfm (cubic feet per minute) of air volume per ton so 728 cfm for 2 tons looks OK. You can ask the builder to check CFM with the new fan setting. You can also ask him to calculate your "superheat" which tells you how the evaporator coil is performing with regards to refrigerant charge. My 3 speed Goodman air handler is set to low. I have all-metal ducts and a 2 story home so low speed works for me. I had to re-read your post to see that it was the return grills that are vibrating and not the supply vents. Has the vibration stopped with the lower fan speed? Best of luck to you.

I haven't tried to reverse it as it is running in the direction that I want. There are no changeable connectors on the motor itself. The only problem that I'm having is that the motor has two black wires running into it and the engine plate with the wiring diagram only shows one. I want to know what the second black wire is for. I get high speed when using either black wire. There seems to be no difference in running.