basic question - stock vs aftermarket shocks

What Boosted said. Plus, the stock shocks don't last very long. They are cheap twin tubes. Might want to think about going mono tube if you do replace.

If you're not going to level then replace them when they go bad. May as well get your money's worth. Leveling doesn't really affect mileage. The mileage get's affected more by the offroad tires people put on after leveling. The advantage for leveling is you can put bigger tires on for offroading and for offroading in general. As for mono tubes. Bilsteins are great and should be more available now. Fox 2.0's are really great but more expensive than Bilstein's. There are others such as King and Icon but they are very very expensive.

If you are not offroading you may not notice anything at all. The difference is that if you swap out twin tubes for twin tubes then they are not going to last very long either. Hence, going to mono tubes which will probably outlast your ownership of the truck. The order would be from least to best would be Bilstein, Fox/Icon, and the best of the best would be King. That said, the cost of mono tubes may be the same as several sets of twin tubes over the life of your truck.