The number series lo-boy tractors actually use the Cub Cadet transmission, not the F-Cub transmission shown above, that is why you cannot find the drain plug. The location of that drain plug shown above is where the hydraulic return line is piped to.
Yea, you have to remove the rear differential cover to drain the transmission. It's not a big deal to do, and I'd get a new gasket from a Cub Cadet dealer prior to draining it. You only have to do it every 5 years or so, so really just not a big deal to remove the cover. It's a good time to spray the inside of the differential out with some diesel fuel to clean out all the gunk. I just did this on a Cub Cadet and it seems that the Hytran gets a little frothy and sticks to the high points inside the gear case after years of mixing with condensation. That stuff turns to a really sticky, goo, that kind of coats things above the liquid level. Anyway, just pop the rear cover off and make sure you've got a big container underneath. It holds about 6-quarts of Hytran.
1951 Farmall Cub, Cub Cadets 102, 104, 1811, 1864, Simplicity Legacy XL 4x4 Diesel with FEL, 60" mower, 50" Tiller