2002 Trans Problems?

I have a 5.3L Automatic 4X4 2002 Yukon with 30k miles. When in 5th gear between 1000 and 1500 RPM's which is usually 50 mph or below I get this noise under the truck. Almost like a clutch engaging and it drops about 50 rpm's. It happens for about 2 seconds and then it clunks. It is worse when going up hill.(Clunk is alot harder and severe). Has anyone else seen or heard of this and what was the fix?

Just got the vehicle back from local Transmission shop. As soon as I told him that I had a GMC yukon he knew right away what the problem was. He installed a 4L60-E trans shift kit and it fixed the problem. I am not at all knowledgeable about tranny's but he said that the main problem is that the converter clutch valve is made out of aluminum and the housing it rides in is steel. Over time the valve wears and does not hold in the lock up position which is why I heard the clunking. The shift kit put a steel one in it's place. Truck is running better then ever now.

how expensive was the repair

$450.00 It takes about 4 1/2 hours to do it. That includes Trans fluid, filter, gasket, shift kit and labor.