I believe #18 is the input shaft seal. If you are going after seals, you must pressure test. If you pressure test, you are looking for any seal that is leaking. Fix em now.
Depending on model drive, the input shaft may require pressing the bearing apart and then back together, then setting the rolling torque on them when it is reassembled. You might post what drive you have, serial numbers, etc. That way you can get specific advice. It is always easier to pressure test and establish the scope of your repairs before buying parts. Last year I had a leaky input shaft seal on my Gen2 Alpha 1 drive. This required buying a torque gauge wrench that was capable of reading single digit inch pounds. As well it required a spanner wrench for the retainer nut. While I was in there I replaced the output shaft seals as well after completely disassembling the gear case. I had already done the seals on the lower the year before. Now we're good for another 20 years.