there are several factors to look at,
what is the total weight of the rig being towed? which model sierra do you have , 1500, 2500 or 3500? the towing capacity for the sierra can be anywhere from 6,000lbs to 10,000lbs depending on options. does the sierra have the heavy duty towing option?
here is a 16 page spec. book for the 2009 GMC full size truck.
http://www.gmc.com/pdf/Sierra09_web_specs.pdf
please take a few minutes to compare how your truck is equipped with the specs in the book and you will be able to determine your truck's max towing cap.
compare this with the total weight of your trailer to determine if it is under the towing limit.
as far as being able to pull the loaded trailer out of the water, a 4wheel drive is better for two reasons, better traction and the ability to use low range, however a two wheel drive with a limited slip rear end should work as long as you can maintain traction on the ramp and the weight of the trailer is under the towing limits of your truck. water depth and ramp angle is also important because you do not want to have to back the rear tires very far into the water.
hope this helps
you can take a few minutes to review