It appears that the rear shocks are indeed the same between FE3 and FE1 suspensions...or at least SERVICED with the same part number (ACDelco 22064727). I'd be surprised if they were the same parts from the factory, but it's not uncommon for service parts to compromise/consolidate applications to standardize on one part. For what it's worth, the front struts are still application-specific (FE1 vs. FE3), at least from ACDelco.
If it were me, I wouldn't mess around with used shocks. Arnott sells a set of new (with a lifetime unlimited miles warranty) rear air shocks that will plug into your computer system for $301. Or buy the whole kit, front and rear, for $601:
http://www.arnottindustries.com/part_CADILLAC_Air_Suspension_Parts_yid6_pid38.html
If it were me, I wouldn't mess around with used shocks. Arnott sells a set of new (with a lifetime unlimited miles warranty) rear air shocks that will plug into your computer system for $301. Or buy the whole kit, front and rear, for $601:
http://www.arnottindustries.com/part_CADILLAC_Air_Suspension_Parts_yid6_pid38.html