Envoy- Check for maintenance records, otherwise the ones we have seen at the shop have been solid. We don't see a lot of them, but the ones we do see are very well maintained and don't seem to have many issues.
Trailblazer- I have replaced a number of front axle shafts on these due to the boots being torn apart, keep up on the maintenance on these as well, we see plenty of these and these are also really reliable trucks. We have also replaced a number of coil packs on these, check engine will/should result when they fail.