GMC 2004 Yokon Denali Owner's Manual page 16

Yukon XL Denali: On the passenger side of
Yukon XL Denali models, the lever at the base of
the seat must be pulled up to release the seatback.
Pull the seatback forward and fold it down until it
is flat.
If the seatback cannot fold flat because it interferes with
the cushion, try moving the front seat forward and/or
bringing the front seatback more upright.
1-10
Once the seatbacks are folded down, on Yukon XL
Denali models only, the rear seat footwell area will be
exposed and will have to be covered by the load
floor panel(s). To create a load floor, do the following:
1. Release the panels from the seatbacks by pushing
forward on the latches.
2. Then fold the panels back to cover the rear seat
footwell area.