Transmission shifts hard when I drive it for a long period.

check out this link http://www.truckmodcentral.com/forums/showthread.php?t=2504 My 2004 Yukon made a clunking noise after I grease the Yoke the noise was gone. I would get a Haynes or Cliffton repair manual, this will make this job a lot easier

It sounds like your transmission is beginning to show signs of wear. When it is cold, all the shift valves that are operated by shift solenoids, have a nice tight fit in their bores. As the trans heats up, everything expands and the shift valves get sloppier in their operation, such as, they stick or they let the fluid pass by them until they suddenly move and the trans goes thump. I would recommend that you visit an ATRA registered shop http://bit.ly/ATRA_Info. Have a good tech drive your Yukon. Don't go to a chain, go to a good independent Tranny shop. They may recommend a flush ( filter and fluid does not do much to get out the sintered metal from the years of wear which can cause shift valve to stick etc, ) You may also have some shift solenoids that are not working properly when they heat up. I have replaced GM shift Solenoids more than once. But, have a good Trans Tech drive your Yukon and evaluate the situation first hand.

I would try this first : remove the driveshaft and grease the Yoke and check the condition of the front and rear Universal joints. Check out this web link http://www.truckmodcentral.com/forums/showthread.php?t=2504 If your able to get hold of a Haynes or Clifton repair manual , , ,get it

There is a transmission sensor. It needs replacing when the transmission is hot or driven for long periods, you shut it off then use it again before it cools down. Or after stopping at a stop light and then taking off and the transmission sounds revy and slugish on shifting. It's the transmission sensor not the transmission itself.

Hey There! I used to have the same problem but GMC built a pretty tough truck. I thought it was my transmission going as well but I replaced my Throttle Position Sensor. Its easy to self install and its found on the side of your throttle body. The part costs no more than $50. It electronically communicates with the transmission on when to shift. It is also a very cheap alternative to an overhaul on your transmission. I had plenty of shops tell me oh its this with your transmission or that with your transmission but it was just this simple device. Give it a try and let me know. You'll instantly feel the difference. Good luck