You can't register online anymore, but you can show up before 1:00 tomorrow afternoon to take a cruise across the Mighty Mackinac Bridge with other Jeep drivers in the Second Annual "Jeep the Mac" event. Last year they had about 450-some vehicles take part in the inaugural event and they're expecting about 600-700 entrants.
2013MDSLNGR posted video here of their trip across the bridge in last year's event.
St. Ignace.com has details about the event here if you want to join the fun with your Jeep, maybe start the weekend a little early.
If the Jeep parade isn't an option for you, you can get in on plenty of other activities coming up at the Mackinac Bridge this year - get a rundown here at mackinawchamber.org.