Doubtful that it is the motor or the brakes, likely the planetary gears, remove the outer cover on the planetary, you will be able to see all of the gears and any damage should be obvious, if you do find damage you can remove the gear set and then should be able to move the sprayer, it won't have much power because the free wheel motor is going to take all of the oil from that circuit because it will spin easier and little to no oil will go to the left front wheel, it will be driving only from the right front and left rear wheels.

If the problem would happen to be the motor is locked up the same procedure will work to move the sprayer but you don't want to run it any more than necessary to get it out of the field to avoid additional contamination to the hydrostatic/hydraulic system.

I find it unlikely to have one brake lock up especially from a failed solenoid because there is one solenoid that controls the parking brake for both rear wheels. I have seen solenoids fail or come unplugged but it always locks both wheels.