Ok thanks for the advice.
Would an alternative be to apply for another spousal visa for 2 years as we will probably apply for dual citizenship after 3 years anyway.
Thanks
There's no such thing as 'another spousal visa'. That is what her ILR is - Indefinite Leave to Remain based on her marriage to you.
A naturalisation applicant must have had ILR for at least 11 months prior to applying to naturalise.
If you're set on going to the US, your best bet would be to apply to get her ILR in person before she leaves. This, as peedal mentioned, is going to cost you, but it's the only way to assure she has her passport in time to go. If the application is posted, the Home Office cannot guarantee a processing time and/or return of documents time.
It might, however, be cheaper to rearrange your travel plans.