Thump in the shifting after engine is warm

We experience a hard shifting, or thump in the shifting, when the engine is warm in our 2000 Chevy Suburban LT 4x4. It has been doing this for years and I have just been lazy in getting it looked at by a mechanic. We had one guy try to pin-point it early on, but he said he did not notice it. Perhaps the engine wasn't warm enough? We have had the transmission fluid drained and replaced a couple of years ago, but it still does it. Could it be the differential? It seems to be shifting hard near the rear of the vehicle. As you can tell by the low mileage, this Suburban is driven by my wife in the city mostly. It has been on a few long road trips over the years to Dallas, San Antonio, Galveston, Fredericksburg, etc. We live in Austin, TX. We used to own a boat that we pulled around occasionally as well (5-6 times only since it was in a boat slip mostly). Any insight on the rough shifting?