My 1996 yukon came with p265/75r16 tires. could i

Should have no problems at all. 245 is just 2 cm ( about an inch ) narrower than a 265. Overall diameter will be slightly smaller as well which will affect your speedometer but not too much. Spedo will read slightly higher than actual speed.

The slightly smaller diameter tire should actually work better for you in normal "on highway" driving.
Your center of gravity will be lower,acceleration will be and feel peppier and the brake feel will be improved.
Because of the "very high ratio" of the final drive in the rear end you might just see a fuel gain with your new "lower ratio" because of the shorter tire allowing less lugging of the engine at lower speeds.
All this for a difference of an inch.
A good change....unless you intend to spend a lot of time off road,then the increased footprint and clearance with the bigger tires might be an advantage.

Yes you can, you won't have any real noticeable problems but the tires you have now are designed to go with the wider rim you have on your Yukon. Installing the tire size you want will cause the tire to "balloon" out just a little bit. Since the rim determines the width of a tire, if you install a narrower tire like you want, the tire will expand out to where the rim wants it, sometimes making the tire look like it's a lower profile. If you want to be sure, stick with a tire that designed to go with the size of rim width that you have (probably 8").

According to ( 1010tires.com ) tire size calculator, they do not recommend any difference above 3% due to possibility of brake failure or brake problems. Your difference calculates out to 3.73 %. Check yourself if you wish.