Ten years ago, I did the same thing with my Case, model "D" tractor. It had the original factory fill gear oil in the tranny which, aside from being very old, was contaminated with water.
I drained all ten gallons of gear oil from the rear end, filled with a 50/50 mix of diesel fuel and used motor oil. I then drove around the farm for about twenty minutes, drained out the mix and refilled the transmission/rear end with ten gallons of used motor oil, (yes, I had been saving this oil for just this reason). I drove around again for about twenty minutes, drained the used oil from the transmission/rear end and refilled with the proper gear oil.
Ten years later, Mighty "D" is still running strong.
With a wheel horse, you might be able to jack it up and during your "flush" simply let the wheels spin with no load whatsoever. Be sure wheel horse is secure so it doesn't fall, hit the ground running and run over something, (like you or anyone else who might happen to be there). Also, be very careful of the spinning wheels. As I recall, the wheel horse tires are a chevron design. If one of the chevrons grabs you, (i.e. loose clothing), it WILL pull you in. That would be bad.
I do want to point out that you are taking a gamble by doing this. Depending on age/condition of the machine, it might be on it's way out and this would speed the process. Obviously, it's your call.
Just an additional note - make sure your tranny lube system is restricted to the transmission itself and does not circulate in any other part of the machine. If it's a typical manual transmission that is a closed system, you should be okay. If it circulates in other parts of the machine, (some "Deutz" tractors are like this), then I would be wary of a "diesel" or "kerosene" flush).