Just some thoughts on a snowy day in Atlanta, GA.
Roof Rack: I'm pretty sure that a roof-mounted tandem rack (e.g., Yakima Sidewinder, ATOC Tandem Topper, Thule Tandem Mount) on an Expedition -- and the 4x4 version in particular -- may create some overhead clearance issues, i.e., they end up being too tall to fit under a standard overhead home garage door as well as many parking decks. Standard, single-bike mounts don't seem to be as much of a problem which is what we used on our '97 Suburban: http://home.att.net/~thetandemlink/a...artopping.html
Unfortunately, our '02 Z71 Suburban is about as tall or taller than a 4x4 Expedition so, alas, no more roof-racks.
Interior Portage: This is how we have hauled our road tandems around for the past 7 or 8 years; wonderful for protecting the bike from 80mph power-washing, hail, dust, and bug blasting and far more convenient than hauling your tandem into a motel -- or worse yet, a hotel -- room for the night. Obviously, the Suburbans can easily swallow-up bicycles and tandems: http://home.att.net/~thetandemlink/a...burbantote.jpg
In fact, that's the only reason we ended up purchasing the '97; of course, now we're hooked. In addition to easily accommodating two tandems and four adults inside for short trips, it has been (before the '02) very easy to throw 3 tandems on the roof and 3 couples inside for long-road trips to the more distant rallies. Less I digress, before the Suburban I carried our tandems in the back of a shell-covered, standard bed Toyota pick-up. Like the Expedition, the rear bed was not long enough to permit me to put the tandem in with the wheels on. Rather than laying the bike in the back on its side or leaning up against the bed wall, I purchased a used, old-style Yakima tandem mount and bolted it to two 1" x 4" pieces of lumber. This allowed me to put the tandem in the mount outside of the truck and then lift up and slide it in, secured in such a way that it stayed upright and would not move.
Putting it on the mount worked quite well:
1. Put the rack on the ground at the rear of the vehicle.
2. Remove the tandems front wheel and secure it to the tandem mount's front fork skewer.
3. Lift the back 1/2 of the tandem up and remove the rear wheel.
4. Place the tandem's boom tube in the mount's boom tray and secure with integrated cinch straps.
5. Pick the whole thing up and slide into the back of the truck.
Rear Racks: They work well, but you may find that your tandem ends up being extra filthy from road grime being sucked up behind the vehicle and some folks seem to struggle with access to the rear lift gates of their vehicles, even on the models that have swing-back features. In fact, one couple who were initially thrilled to death with the Draftmaster on their barn-door equipped Tahoe, gave it up after the first year because of the interference issues. They opted to remove their wheels and carry their tandem in the back of the Tahoe for the above mentioned reasons.
Bottom Line: There are many, many different ways to carry your tandem and equally as many different vendors offering solutions for the various ways. Evaluate your needs, your budget, and picture how you'll use what you think you want before laying out any cash. In the mean-time, an old blanket will work wonders at keeping the inside of your Expedition from picking up any grime on your tandem...