I've seen Avalanche's with EXT gates installed - so they will fit an Avalanche. ?But the body line across the bottom wont line up nicely with the tail lights like the Av's does now. ?The tail lights are different between Av and EXT.
Also the top plastic trim is different, but I guess that you should be able to transfer from the Av 'gate over to the EXT. ?Tailgate handle and rear handle Bezel are also different, but swappable. ?I've seen Av's with the EXT bezel and handle and the Av top trim.
Just search a little for EXT trim, EXT tail gate, or EXT Tail lights for some pictures - they're on the forum somewhere, I've seen them.