Easier than if it's in, but not much different. It will require pulling the pan, so with the tranny out you'll need to support the engine from above, so you can pull the pan. Most 2nd gens will require the motor mount bolt to be removed so you can lift the engine enough to remove the pan.
Once the pan is off then it's removing the rear main cap and if it's a 2 piece seal you can work it out with a seal puller, then push the new one in around the crank. You have to be careful not to shave any rubber off the seal when installing, or it will leak again.
On a one piece seal you need to drop the crank to replace it, so I usually pull the engine out to do them; just lots easier for me.

 

damn, sounds like way too much work for a seal. i have been getting screwed by shops lately so i wanted the guys at the transmission place to do it since the seem to be a pretty honest group. i'm sick of having cardboard all over the garage floor you know?