My 2004 Envoy does not seem to shift right on the highway
With the limited information you provided, it would be a good idea to bring your Envoy to a transmission shop for analysis. Most general repair shops and even many dealers do not do this work because it is so specialized. Bringing it directly to a transmission shop yourself will save the "mark up" charged by general repair shops.
You can expect problems with the transmission if you do not have it serviced regularly. Problems can be intensified if you pull a trailer, drive in hot, dusty areas, or do a lot of stop and go driving and do not perform the scheduled maintenance.
Sometimes, low fluid level can be a source of the problem and other times a change of fluid can solve the problem - but not as often as one would hope. Check the fluid level on the dipstick. Add enough fluid to bring it to the correct level. The fluid should be pinkish/red in color, but will likely be darker if it has been in for a long time. Regardless of color, the fluid should not smell burnt. An improperly shifting / slipping transmission coupled with burnt smelling fluid is an indicator of a potential expensive transmission rebuild on the horizon.