I'm trying to upload a replay on the 360 version of Last Round. I'm getting a message that says I can't upload because "my grade isn't high enough". Does this mean I have to rank up on Rank Matches first? Kinda weird.
You are making a misunderstanding, this is the wrong kind of uploading: you are selecting the option that allow you to include a video about your performance when people check the worldwide ranked list of players and see your placement. That option is allowed only to people that have at least a certain rank (maybe S or S+? I don't know for certain).
If you wish to upload a video on Facebook (on 360 you can't do it on You Tube), you need before to go on the online options of the game, set "yes" on Facebook account, and then give your mail and password associated to it. When this process would be completed (it's possible that you will need to give the confirmation more times becuase not always works at the first), later you will see a fourth option for each one of your recorded video, the one to upload them on Facebook.
Also, keep in mind that from the end of April will not be more possible to upload videos directly on You Tube or Facebook for both the PS3 and Xbox 360 versions of the game (and PS4 and Xbox One don't allow it).
[Important Notice #2] Due to a Facebook policy change, you will not be able to use Facebook share and YouTube uploading functions (Continue)
from "DOA5 Plus", "DOA5 Ultimate", and PS3 / Xbox360 version of "DOA5 Last Round" from 4/30/2015.
The PS4, Xbox One & Steam versions of "DOA5 Last Round" do not support the account linking functionality for Facebook and YouTube. #DOA5LR
— Team NINJA (@TeamNINJAStudio) January 13, 2015