I feel I'm closing in!
If so, when you talk to him about the wreck, Prick will offer to go to Bobbi's place and get a tow truck.
Once you've met Farmer Brown, you can borrow the tractor and save Maibot. After that, just go back to the Postal Bunker and talk to the sentries again. They'll let you in just like they promised they would when you offered to go find Mailbot. Take the package to the cow and the rest should fall in place!
Hope that helps!