I didn't know we'd need a sponsor letter. Does this mean the UK sponsor (i.e., husband in this case) would provide a letter and send a scanned copy to wife in US? Should this also be dated prior to application submission?
The sponsor letter is absolutely mandatory - the visa can be refused without it. The letter should be original, handsigned and submitted along with all of your supporting documents (i.e. either physically mailed to her in the US, or submitted in the UK by you if that is now an option).
Now that your application date has passed, you really should date it with the current date, because to back-date it would be lying.
The sponsor letter needs to include:
Paragraph 1: how you qualify to be a sponsor (your UK citizenship and history of living in the UK)
Paragraph 2: how you meet the financial requirement, which financial category, your current employment details, how long you have been with the company, current salary and how long you've been earning that salary
Paragraph 3: where you will live together in the UK, including full address, who owns it, property details and stating it will not be overcrowded
Paragraph 4: short, factual history of the relationship, including how, when and where you met, when/how you entered into a relationship, when/where you got engaged and married (no personal feelings just dates and locations) and explanation of why you are choosing to settle in the UK together.
Just a tip, in case you are worried about getting everything together in time, as long as your wife has NOT YET attended her biometrics appointment, it is still possible to cancel the application and request a refund (not sure what happens to the priority fee though). It can take a few weeks for the refund to come through though.
If you need to, you can do this, then spend some time getting all the documents together and checking they are all in the correct format (as the requirements are very specific), and then you can apply again when you have all the documents completely ready to go.