I had the LTX MS2's on my 08 Envoy. I went through two sets with never a problem. Anything can happen in a 1000 miles though. The tires are the easiest thing (and usually rightly so) to point a finger at when ride quality diminishes. If it's a tire problem, Michelin will stand behind it and make an adjustment. What more can they do? But there's a ton of other things that can cause poor ride quality/vibration. It could be front end wear and tear, shocks, bearings, loose lug nuts etc. The first place to check would be the wheels and tires though, after determining that there are no wear patterns caused by misalignment, lack of rotation , suspension or other mechanical problems. Even improperly torqued lug nuts can create problems.
If that vibration keeps coming back after a while it could be that they are using the wrong type of wheel weights. Ask then to double check this. Depending on your wheels, there are different types of rim flanges. Steel rims and alloy rims usually take a different type of wheel weight. Sometimes the installer will not use the right ones. Some wheels can not take (usually for cosmetic reasons) the type of weights that are hammered on the rim. In that case they use tape weights on the back side of the rim. If the incorrect type of wheel weight is used it can fly off or shift while driving or mudding. If you like to spin your wheels, that can cause the weights to slide on the rim. If they use tape weights, they need to properly clean the road grime from the rim first. Another problem with tire installers is that sometimes they will forget to remove the old weights before balancing. Then they wind up chasing weights around the rim. It will balance in most cases but it's ugly and shoddy. Of course, there could be a rim that is slightly bent as well. Bent or damaged wheels can occur at any mileage so maybe you want to make sure they are true. A good tire buster can tell quickly if that's the case. Even a slightly bent rim can lead to ride quality problems. In most cases, mechanical issues will put bad wear patterns on a tire and will cause that tire to appear bad. In reality, it is bad at that point but it wasn't the tries fault. A new or replacement tire will begin to wear in exactly the same way.
Your mileage may vary.
Cheers.