This is a pic of a twin-stick d20 behind a t-18 for reference purposes.
To remove the rear output yoke, simply remove the nut. You May find a serrated rubber washer under the nut that is for the purpose of sealing the splined area preventing oil migration through the splines.
The yoke will simply pull right off the output shaft spline though you May have to help it. Inspect the machined area of the yoke for damage from the seal, if it's grooved, the yoke needs to be replaced with new, or a repair sleeve installed on the yoke.
The seal May be popped out using a seal removal tool or a long, heavy screwdriver/pry tool. You May also find a felt ring slid over the shaft, or some type of thin sheet metal "slinger".
It's easier to remove the seal safely by removing the rear bearing housing and driving the seal out. But you will also need to replace the gasket and you "might" find shims between the bearing retainer, the gasket, and the housing, those are used for bearing adjustment, not all cases will have shims at that location.
If you remove the bearing retainer, use a new gasket at that location, do not use rtv! That gasket acts as a shim/spacer for the bearing/output shaft.
Ihon has the gasket and seal kits in stock, along with separate output shaft seals.
