Don't forget about weakness' in the 4L60-E tranny. I would agree with everyone esle to change out your u-joints first. The click or clunk as you describe it, should be the first sign that the needle bearings in your u-joints are at their end of service. Cheap piece of mind. Change them. Also check out these two sites for 4L60-E Tranny problems and solutions should you encounter either down the road.
http://www.thesurecure.com/parts/parts/SC-4L60-E.htm
http://www.transmissioncenter.net/ex...duty_parts.htm