The idea of jacking the trailer with another jack may be a first attempt. Take the load off the trailer jack and attempt to lower the trailer jack with the electric switch.
What has happened is you have jammed the shaft in the jack and the motor is forcing against it and releasing against the safety clutch. By removing the weight load on the jack it may be enough to allow the motor to move the jack shaft.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles