The live rear axle will always be a favorite among traditionalists, which is fine, nothing wrong with that at all. They are simple to maintain and generally easy to work on.
The independent rear suspension (IRS) did offer several advantages over the live axle though.
One was packaging (vehicle design) allowing the third-row seats to fold flat "disappearing" into the load floor. A priceless advantage in itself – no more having to remove the third-row of seats for additional space. Some of the Expedition’s competitors (I shall not name names) still can’t do this party trick.
Second was vehicle stability and handling. The IRS allows for more precise wheel control and suspension tuning.
The 2003 Expedition (U222) was the first in its class to feature a IRS, which is something to be proud of. Very advanced and highly controversial at the time. The Nissan Armada and Toyota Sequoia have since copied it. Imitation, they say, is the sincerest form of flattery.
U222’s IRS with coil springs allowed for a remarkable 9.4 inches of long suspension travel, (note opting for air springs reduced that number) and obstacle clearance improved by 2 inches (50 mm) under the axle differential.
The control arms, shafts, bushings, etc of the IRS were beefed-up to handle higher trailer loads, as the tow rating for U222 increased by 800 lbs over the first-generation Expedition (UN93).
The IRS was re-designed for the 2007 Expedition (U324 and U354), getting more heavy-duty parts, a five-link arrangement, and new lower control arms.
The next-generation 2015 Expedition will stay with a IRS system. The 5.0 liter 32-valve DOHC Ti-VCT Coyote V-8 is confirmed (so the Expedition is getting it) along with a 10-speed automatic transmission.