If you live in the snowbelt then I'd suggest getting the locking differential.
Issues other Envoy owners seem to have: excessive wind noise from front doors (dealers say it's normal), roof rack wind noise (TSB regarding placement of rails helps alleviate this), engine cylinder sleeve problems on 2001 models (GM extended the warranty for some 2001 models).
And be sure to test-drive the final vehicle you choose... we test drove several from the dealer inventory, but purchased one they had transferred from another dealer. We didn't test that one; and regret it. (It has a "stutter" or "hiccup" at idle in drive when at a stoplight for over 30 seconds). I'm disappointed that GM isn't taking responsibility for the problem. It's in the BBB hands now.