I have a '99 Tahoe, 4wd push button style, 5.7 Vortec with 110K that's been very well maintained. When I'm pulling up a hill, and it shifts into 3rd gear and hits 2,500 rpm or more, a heavy vibration occurs. I thought I had a flat tire or something went terribly out of balance. After slowing down or stopping, everything works just fine until I encounter the same circumstances again, and it doesn't happen every time. I pull a 3,200 pound travel trailer occasionally, and this is when I notice it most, because that's when I reach 2,500 rpm and 3rd gear, but it has happened twice without the trailer. It doesn't matter if cruise control is on or off. I don't use the CC when I pull the trailer. A transmission service center checked it out with a computer while driving it for 1/2 an hour and found nothing. I had a complete engine tune up, but the issue was not resolved. This has been going on for 6 months. Everything works smoothly until I get into 3rd gear @ 2,500 rpms or above pulling a hill. It has never happened below 2,500 rpm or in any other gear. Please help. I don't know where to go from here.