Hi,
I don't know if I will be much help, but I can tell you this. Your problem lands between your front brake rotors and your driveline. Are you certain that it ONLY happens in reverse? Manual or Auto? If only reverse I would suspect the trans because that is the only variable in my experience. If that is the case, you better get it TOWED to the dealer if you only have 40Km left on your warranty! Since we are talking Kilometers, that means you must be from Canada-land like me (America's 'hat'). Are you in Ontario? If so, where abouts?
Anyways, I know someone with a 2000 Focus sedan with a 2.0L Zetec, auto, and she has a bad vibration in Drive. I never tried reverse, but now I am curious!
I have seen a bad vibration on a Dodge Caravan at idle speed in gear due to a cracked flywheel (around the mounting bolts at the crank). I found out the problem after I told him to send it to a shop, and that is what they found. I inspected it myself before-hand, and determined the problem was likely engine-internal, which it wasn't. Along the same lines though, it was driveline internal. I checked with my hands on the motor/trans and with a mechanics stethescope. Anyways, I want you to be very analytical.
If it ONLY happens in reverse, and is automatic, then it is likely the tranny, flywheel OR torque converter, but not likely the torque converter. If it is manual, then it is likely the trans.
Also, there are 3 physical mounts that mount your eng/trans as a unit. One on the passenger side (motor mount), one on the drivers side (trans mount), and one at the bottom rear center of the assembly that bolts to the undercarriage to stabilize any deflection (I don't know what they actually call it, but it is about 4 or 5" long and has one bolt on the eng/trans assy, and one bolt on the chassis of the car. Easy to see if you are under the car looking up at the firewall.
Anyways, I hope this is helpful. Feel free to give me more detail or ask any questions. Let me know how you make out!
Kev