anyone travel to Mexico for surgery w/o passport??

Per the state Depts website: 

If you are Traveling within 14 Days or Need a Passport for a Foreign Visa within 4 Weeks and:

• Have not yet applied, see Schedule an Appointment to submit your application at a Passport Agency.
• Have already applied, see National Passport Information Center to contact a Customer Service Representative when you are 7 days out from your travel date.

http://travel.state.gov/passport/processing/processing_1740.html
I'd check with them and see what the requirements are to get your passport back on time.  I had mine expedited and it came extremely quickly, well before the 2 weeks.  It even came before the website to check the status on it shows that it was processed.

You will definitely need it, even if you are in a clinic van.  We all had to show our passports before getting back to the US. 

Also, I was told when getting my passport (originally  I was only going to get a passport card), that the card is only for driving through border crossings.

If, God forbid, I had to be life-flighted out, I'd be SOL with just the card, you need a passport for any flights, even emergency life-flights.

If you have no other choice, just take the passport application copy along with the receipt showing you paid for your passport(along with birth certificate copy and license) and you should be able to cross back over. It's nothing I'd chance unless I didn't have another choice. Thankfully our expedited passports came in time.

It's still 18 days until your surgery.....you have plenty of time to get it expedited.  Do not go there without it.