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Disassemble Drive Shaft Housing

This video shows separating the Drive Shaft Housing from the Gear Housing, disassembling the Upper Half, removing the Shift Assembly and removing the U-Joint Assembly. You will need the large spanner wrench to loosen the Ring Nut. It is nice to see what it all looks like before beginning the work yourself.
Maybe you don't have a fancy bench just like this one. Just rig up something sturdy. A sheet of plywood on two sawhorses makes a nice large table to lay out all of your parts. Keep your Service Manual there too.
Click that CC (Closed Captioned) button at the bottom of the video over there and read the text. Be sure to unclick the option when you watch some other videos or you may see some text in really poor English as it could be computer generated.

Water and oil sludge in the shift linkage cavity/rear cover.
This starboard unit had either a leaky seal washer in the bell housing at the end of the shift cable (item 4. B. Page 4A-33 Mercruiser Service Manual #28), leaky o-ring (item 22 Page 3A-13 Mercruiser Service Manual #28) or leaky rear cover seal. It definitely had a leaky shift shaft seal. I brought the boat home, winterized everything and planned to pull the drives the next morning. The temp dipped to 20 degrees that night. The water froze and cracked the cover. Water and oil could be seen leaking out of the cover in the morning. A new cover is around $300. I sniped a used one on eBay for $15. There was no other damage. Lucky. I can see that the Previous Owner had replaced the cover at some point. It was the only part of either outdrive with a decal.

Disassemble Drive Shaft Housing

If you watch this video on my youtube channel, you can turn on Interactive Transcript or just click on CC for Helpful Hints and Commentary.
Oil Monitor Bottles
I pulled both units. The port side had oil in the bellows due to a leaky Drive Shaft Seal. The port drive ran low on oil and the Drive Shaft Bearings were overheated and needed to be replaced. It could have been worse, I guess. This boat in 1988 did not come with oil monitor bottles. Both Transom Assemblies, Bell Housings and complete outdrives were replaced by the Previous Owner. The replacements are newer (2000-03) and have fittings for the oil monitor bottles. The bottles were not installed. The lines were just capped off inside the engine compartment. If the bottles were installed, the oil could have been replenished and the bearings would not have been damaged. I have since installed new monitor bottles. I wired the new monitor bottles to new LED indicators I installed at the helm. The monitor bottles could be connected to the Mercruiser water/oil alarm. But, that's not as cool.

Upper Gear Housing Seal Kit
For Mercruiser
26-16709A2, GLM 87480 - Sierra