Hello everyone new to the forum and new to a GMC truck I have 2 other Ford trucks trucks. just have a quick question . My 2009 GMC Sierra with the 4.8 liter is almost exclusively used to tow a 6500 pound trailer and I was wondering if there was a way to lock out the overdrive because it is constantly hunting for gears while towing. Thanks to everyone in advance for any help that can be offered.

I would take a good look into your owners manual. If you don't have one, often they are available free online.
I believe there is a tow/haul mode on them that probably changes shift points and may lock out the OD.

I was going to say, does it not have a Tow/Haul button on end of shifter?

yes mine does have a tow haul button and I use it but it does not block out the overdrive. I have discovered that if I leave it in 3rd it locks out the overdrive as like with my fords if I put it in the two or three gear it starts in that gear and stays in that gear but I see with GM it is different. So I guess problem solved

6500 lb trailer, I hope you have an external trans cooler.

no I don't but I only pull it 10 miles one way last time I pulled it the transmission got to 203

I guess that'* OK?

Yes you should be ok when pulling the short distance then.
203F is just nicely warmed up.
How often do you pull the trailer, occasionally?

203 is kinda ok. Granted, its a short run. But, anything less than 200 is ideal.

You should be able to shift to '3' to keep it out of overdrive, which, assuming you have a 4 speed transmission (I think only the 3/4+ ton had the 6 speed), will accomplish exactly what you are looking for. Ford is notorious for calling it Drive and Overdrive, as well as older GM as well, but now 3 is the same thing, it stops shifting past 3rd gear.

If it'* not, then there is something wrong and we can help track it down.