"...flying out of Guadalajara on April 4 and returning on April 8th. My current US passport expires on April 10th, so I'm afraid that Mexican immigration would deny my flying back to Mexico with only three days (including flight day) left on my US passport. "
I don't see what your problem is if you are returning to MX with a passport that HAS NOT EXPIRED. Going to "expire in 3 days" - so what, it won't BE expired. I would suggest you take your trip as planned. Make appointment with GDL on the 9th and apply for renewal.