Tami -
Have you considered carrying your bikes on a rack on the liftgate of the Navigator? We have an Allen 103A, a strap-on rack that lets us carry our bikes on the back of our Explorer. We have carried our two bikes at least 10,000 miles - for at least 2000 of those, we actually had three full-size bikes. The rack goes on and off in a hurry - you simply drop two clips into the gap at the top of the tailgate, and tighten a lower strap around the stem of the trailer hitch. When you are not using it, it folds flat and is easy to store, so there are no theft problems. It is not expensive - and it keeps the weight where it belongs. It also keeps you from inadvertently backing your TM too close to a tree or a wall, and bending a couple expensive bicycles! The only drawback is that you can't open the tailgate very far when the bikes are in the rack.
http://www.allenracks.com/products/trunk/103a.html
There is a better picture of how one of these fits on an SUV at
http://www.allenracks.com/products/trunk/143a.html
Bill