I have a Denali and the transmission was rebuilt by GM dealer when it had 88000 miles on it. It has been driven about 40000 since it was rebuilt and is starting to slip between 2nd and 3rd gear. Is there a reason my transmission would be slipping. I do not tow with it...Very frustrated

There are a few different things that can cause this. It could be a shift solenoid or even the ignition switch isn't supplying enough amperage to the transmission harness to properly operate the electronics in your transmission. It could be the pressure control solenoid going bad causing internal transmission failure. The 4l60e transmission is generally a good transmission but I have seen them fail as in less than 20k miles and I have seen others with over 200k and never been touched. I suggest getting it scanned and see if it had any stored transmission codes and go from there. You might get lucky and only have a minor electrical issue but the more you drive it with it slipping the more internal damage your doing to the transmission.