Technically they are not the same, due to the amount of differences.
The platforms (chassis & suspension) are not the same for the three different generations.
The first generation Expedition (UN93) used the original U platform. The second generation Expedition (U222) used a all-new redesigned U platform, sometimes called U2. The third generation Expedition (U324) and Expedition EL (U354) use the all-new T platform, also called T1 platform or simply T1.
The four-door body (five if you count the liftgate) was carried over throughout the years, but not without heavy modification.
The first generation Expedition (UN93) has the original body, naturally.
The body of the UN93 was reused for the second generation Expedition (U222), however its was completely redisgned from the "A-pillar" forward (front end), and "C-pillar" rearward (rear quarter panels, glass and liftgate). Only the mid section containing the "B-pillar" was left largely alone.
If you look close, and even measure the rear quarter-panel window glass cut out and glass shape of the U222, you will find its different from the original UN93.
The doors are the same shape and have the same door handle cut outs (though the door handles themselves were redesigned), but even they were reworked slightly for better sealing when closed. So they might not work on older UN93 bodies.
The body on the U324 is the same as the U222, but has thicker side window glass and an acoustic sound deadening windshield. So while the glass is cut the same shape as the older U222 and UN93, its actually of all-new material and construction.
Of course there are other small differences like moldings, mirrors, roof rack, interior designs, and so on that changed over the years.