Sorry to hear about that.
That's a lot of miles on a tk, and an 08 isn't that much newer than what you have.
Have you had your truck's trans fluid changed lately? May not help, but listen to my story that happened to ME: About 11 years ago I changed fluid on my truck's trans. I was using NAPA (Valvoline) ATF at the time. About a week later it started slipping and going between 3-4 constantly ...really JERKING the truck, felt just gawd-awful! I thought I was for sure going to have to have it rebuilt. Just before pulling the trigger on a new tranny, I came across some posts online where guys had something similar and had success resolving the issue by changing the fluid to Penzoil ATF, because Penzoil has a different hi-mileage additive package with more anti-shudder agents. Some fluids may not play well with the friction plates. I thought it was a too-good-to-be-true thing, but I figured for $25 it might be worth a shot and maybe I could get another 6 months or year out of it. Anyway, the symptoms went away in about 50-75 miles and never reappeared. That was 11 years ago and I still own the truck!
So ...worth a try, depending on what they come back with. It might be a solenoid too (I had that with our Jeep), or wiring. Don't panic. Mechanics often miss 'root' problems, so I'd take it to another shop and a different mechanic (like the GM dealership). If the tranny shop tells you it needs to be rebuilt, I mean, shops WANT to rebuild or replace your transmission! That's good revenue. Again, don't panic ..people who panic generally make bad decisions. If you DO have them (or someone else) change the fluid, once it's done (but BEFORE they start the vehicle), remove one of the tranny cooling lines (at the radiator) and run a hose from the line into a 5-gal bucket. Then add two more qts to the tranny and have someone start the truck while you watch the bucket. Once it starts the pump will quickly pump fluid from the torque converter through the cooling line and into the bucket. Once you see the fluid go 'clear' shut the vehicle off and top off the tranny. If the fluid was recently changed, then just count to 7 seconds and shut off. Then you will have exchanged ALL the fluid!
If that doesn't do it, add some TransX and see if that brings it back (I've had that work too). Then you can be more methodical in your search.
That said, my truck with a 5.7 has 150k and runs great and feels tight. We have a 4.7 in a Jeep Grand Cherokee with 200k mi on it and it runs fantastic still, no excessive oil consumption, noises or anything. The headgaskets were changed a long time ago though. But OTOH, I don't put that many miles on my tk per year and can do my own maintenance. If you put on a lot of miles, in a couple years you might have 240k miles on that truck and thats REALLY getting up there and asking for problems. You might be back in the tranny problem situation or expect head gaskets ..things like that. You never know. If you can't do much vehicle maintenance, I'd try to find something in the 80k or so mile range. Just sayin'
