It Is Not Required
The short answer is that you do not need a passport to get back into the United States from Mexico. However, traveling into Mexico without a passport may cause an increased wait time when you walk back across the border into the United States. Bringing documentation that proves your citizenship (e.g. a Birth Certificate) is suggested. The more documentation, the better.
Great news, if you have a passport, that is all of the documentation you need, just bring it with you. While passports are not required to re-enter the United States from Mexico they do greatly expedit the process and are highly recommended.
If you do not have a United States passport, but are nevertheless a citizen of the United States, you can still come to Baja Ibogaine in Mexico.
A Note About Mexican Visas
When you drive into Mexico, you are not usually stopped by immigration. This means that the Mexican authorities do not ask to see your passport. However, when you walk thru the border, non-Mexican entrants are asked for their passport. You cannot obtain a Mexican visa, which is technically required, without a valid passport. As long as you are not stopped at the border when entering Mexico, this will not be an issue.