Shakes or Wobbles problem
2006 Chevy Silverado V8 Four Wheel Drive Automatic 100000 miles

6.6 Diesel. My truck has developed a vibration under load that is sometimes constant and sometimes goes away at various times. It also vibrates at idle sometimes. I had it at the dealer thinking that it was the u-joints but they found nothing. I would have assumed that they would have checked the motor mounts as a next step but didn't. The mechanic that I usually take my truck to is 100 miles away and I didn't want to take the chance of driving that far. By phone call he suggested it might be a motor mount. He has always been able to find problems in the past and take care of them. He's about the only one I trust. Are the motor mounts difficult to change? I can fix most things if I know what's wrong, I'm just not a good trouble shooter. Thanks in advance.