I'm surprised that there is only one switch. Doesn't make much sense that they would set it up that way.
Possible causes for different speeds:
1. Voltage difference at each motor. Voltage drops could be occurring from bad connections, worn solenoids, corroded cables etc. You could hunt this down with a digital volt meter. I think this Is the most likely issue.
2. Different condition of the two motors. I'm not sure if these motors have replaceable brushes...if so then new brushes for both of them might help even things out.
3. Differences in friction between the two. Seems unlikely that there would be a difference in friction at the outdrive. Not likely at the lift cylinders either if they look like they are in good condition. Contaminated Hydraulic fluid (is one milky)?
4. An internal leak in a lift cylinder. If the O-ring on the piston of the lift cylinder leaks, it will allow that drive to "leak down" without any oil loss. You did not describe any leak down symptom, so I don't think you have that problem.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings