Do I need to replace just the back two (2) tires? Could this be the reason for the high fuel consum.

You no not necessarily need to replace all 4, but an AWD vehicle, the tread SHOULD be near even all around to prevent damage to the drivetrain.

Umm... Just a thought, but if your tires aren't wearing evenly and the worn tire has a 'hump' in it, you probably need to check your tire pressure. Set it to the recommendations on the door stickers. Other than that. You'll want to replace the backs together. Generally on newer vehicles the transfer case has a fluid coupling similar to how a torque converter works, and allows for some slip, which will keep your car from being damaged. However, that being said you'll want to replace the fronts as soon as possible. As for the tires/brand question... Generally you want the same brand and type of tire for two reasons. 1- aestetics, looks goofy having different tires on front and back, and 2 the different tread types will give different noises at different speeds (which is annoying) and grip differently in weather (IE slips in rain but grips in snow, back vice-versa.) Long story short, get all four if you can, but get the back two first if that's all you can afford and get the fronts later. And check your tire pressure