Actually has nothing to do with the X-REAS, as you can get the limited with the X-REAS with the 3rd row. Has to do with what they wanted to release. Sports Edition doesn't sound right with 3rd row, and for a while they were releasing all Sports Editions with the 10 speaker system (which eliminates the seatbelt anchor for the 3rd row) and the sunroof (which also eliminates the 3rd row). If you have a sunroof, reduce the headroom for the 3rd row from 32 inches down to about 30 inches.
If you wanted to tear out the two pillar speakers in the rear, tear out the rear plastic sidewalls, install anchors into the floor and sidewall areas for seatbelts and seats to bolt into, the order the plastics, seats, and seatbelts, you could probably do it.... The real question would be how much are you willing to spend to get that? say another 4,000? That's probably what you will spend getting all of the stuff together to re-fit your truck.