I purchased a 2000 Yukon SLT a few months back and had this same issue. Took it to several shops and the dealer and all gave me different ideas of what they thought it might be. So I took it on myself to figure it out. Here is what I discovered. Noise and vibration only under load(pushing gas pedal) but when off load noise and vibration would stop. Everyone kept telling me it was drive train related but I did not think so. While under the truck, with the front splash shield removed I noticed the noise was coming from the A/C unit, checking further I noticed the A/C clutch had something sticking out of it and discovered it was the hard rubber surround, that had failed and broken loose. (see pic below) I temporarily unplugged the wire harness from the A/C and the noise and vibration was gone. Later that week I got a used part for $3.00 from a local yard replaced and been working great so far for the last 6 months no noise or vibration. FYI the A/C units always runs when you have the fan switched on, even on the heat setting. Was a really quick fix that everyone wanted from $300.00 to $3000.00 for so just an idea, hope it helps.
