Put a jack with a block of wood to support the engine/transmisssion. Pull the driveshaft, crossmember, tranny mount. Remove the bolts that attach the tailshaft to the tranny. It should just slip off the back of the output shaft. Installation is just the reverse.
Procure a new seal and tailshaft bushing. I would take it to a transmision shop (or other machine shop) and let them remove the old and install the new bushing. I've done it myself without the proper tools, and it ain't purty!
Be sure to inspect the U-joint yoke where it contacts the bushing. Replacing the bushing alone will not solve any problems if the yoke is worn.