I have a problem that occurs only when I'm towing. I have a new-to-me 2001 Coleman/Fleetwood Santa Fe popup, I'm towing with a 2002 GMC Sierra 1500 with the 5.3 V8 and AT. It has the factory tow package. Tires are new and recently balanced. Sometimes, but not always, as I'm towing in the 45 - 60 mph range, the engine shudders ... I can feel the vibration through the steering wheel and have a sense of the whole truck briefly vibrating. It only happens when I'm towing my popup, it never happens when I drive the truck without towing. It kind of comes and goes even when it's happening, so it's an intermittent problem, and it doesn't happen every time between 45-60 mph. When I towed it last week, it scared the bejeebers out of me because of a previous event. When I bought the pup last Xmas eve, I was towing it home and the engine did a similar "shuddering" ... just before it quit completely. I got the truck and pup towed home; my mechanic said it was the fuel pump and that GM pickups of that vintage are known for fuel pump failures. So when it started shuddering at the start of my recent camping trip, I was paranoid the whole time that it would die and leave me on the side of the road. But it never died and I made it about 1500 miles without the engine dying. It doesn't happen below 45, or above 60 MPH. It almost feels like there is some harmonic imbalance between my truck and my pup at certain speeds ... but that sounds wacky so I'm turning to the wisdom of this forum for any advice you may have. Ever had to deal with something like this?