[Yosemite Sam]

If you already have it out (the rear end) You can get away with tipping it up to not lose the fluid when you take the plunger out.

Sometimes the plungers can be difficult to remove, they have a taper on the front, outside part of them. I usually use an ice pick to clean the area where the taper is.

If I remember correctly on the last one, I clamped a pair of small vice grips (the ones with the kernelled jaws) on the plunger, one jaw inside the hole where the steel ball goes and the other jaw on the taper, and used a twisting motion to pull the plunger out.

I don't think there is anything special about the "O" ring, you should be able to pick one up anyplace where they sell such things.

If yours is leaking, I wouldn't suspect that it is "frozen" in the hole, but if it is, you may have to remove the reduction housing and drive it out from the back side.

Good luck.