ESTA is only required if you enter the US by plane or onboard a cruise ship.
If arriving by a land border, you don't need an ESTA, but may need to complete a paper I-94W (you shouldn't in your specific case, in theory).
From the official ESTA site
IF I AM IN CANADA OR MEXICO AND WANT TO DRIVE TO THE U.S., DO I NEED TO APPLY FOR ESTA?
If you are a citizen of a country that participates in the Visa Waiver Program (VWP), and you enter the U.S. by land from Mexico or Canada, an ESTA is not required.
First entry via land border without a valid ESTA - You will be required to complete the paper I-94W form and proceed through a manual entry process at the land border crossing.
First entry via land border with a valid ESTA - You will not be required to complete the paper I-94W form, but you must still proceed through a manual entry process at the land border crossing.
Reentry via land border with a valid I-94W or valid stamp in passport from a previous admission – If you reenter the U.S. after a trip of less than 30 days to Mexico or Canada, you will not be required to obtain a new I-94W and your entry process is expedited.