First off (and in general for anyone looking for help) please post your mods! Quite difficult (and a lot of guessing) if we don't know the hardware you're dealing with.
Highly reccomend finding someone with a Tactrix cable (or getting one yourself) and getting some logs. Uncle Scotty's kind of got the method to the madness of diagnosis (follow the SM).
In a generic sense, DS fuel trim lean (P2096) means that the control system couldn't get the downstream sensor down to the voltage it wanted, so the system is out of control. As such, you get a fault. COuld be a problem with the DS sensor reading incorrectly high (sensor/wiring problem, sensor out of exh stream) or a tune/HW issue where you're running a bit richer than the cal is set up for and the DS control is trying to pull out fuel to get you back to "normal" but runs out of authority.
Possible causes in order of SM diagnostic procedure) are:
2) rear O2 sensor reading stuck hi or offset (failed sensor or wiring, should be able to see it on logs)
3) rear O2 sensor offset hi/biased (failed sensor or wiring, should be able to see it on logs)
4) rear O2 sensor wiring failed between ECU and sensor signal (fix wiring)
5) rear O2 sensor signal reference offset (failed sensor, replace)
6) air leak in the exhaust near the front O2 or rear O2 is isolated (repair exhaust)
7) ??? not sure how an air leak would cause this, but a borked MAF might
8) high fuel pressure causing rich event
9) failed pressure regulator (see above)
... (I'm too lazy to check these, they are unlikely)
I'd say an aftermarket exhaust by itself is unlikely to cause this, but any of the various "CEL" fixes applied to the DS O2 _could_ be causing it.....