Looked at Spouse visa and reading more now about settlement visa just to be sure that's not a route to go. We were married in UK if it makes any difference... anyway, I'll post more about that when I can.
A spouse visa is a type of 'settlement' visa if applied for outside the UK (the 'settlement' visa category covers all family-related visas).
It doesn't matter where you got married, it just matters that you are married when you apply for the visa.
If you have been married and living together outside the UK for more than 4 years, you can qualify for permanent residence in the UK almost right away instead of having to live in the UK for 2 years first.
In order to get permanent residence, you need to take the 'Knowledge of Life in the UK' (KOL) test first, which can only be taken in the UK.
So, you have 2 options for applying for your visa if you meet the 4-year requirement:
1) Apply for a spousal visa, and request that you are given a 'KOL' required stamp, based on being married to a UK citizen and living together outside the UK for more than 4 years (you will need to provide evidence of this). This allows you to move to the UK, take the KOL test whenever you're ready and then apply for ILR (Indefinite Leave to Remain) without having to wait until you meet the 2-year residency requirement.
2) Take a trip to the UK as a visitor before you apply for the visa and taken and pass the KOL test while you are there. Then, once you have returned to the US, you can apply for a spousal visa, but request ILE (Indefinite Leave to Enter) instead... if it is granted, this will give you permanent residence right away.