2004 xterra 6cyl 3.3L (160k miles) & 2003 xterra 6cyl 3.3L (dead engine)
This is NOT the 1st time I've been screwed over at a shop, trust me I've been around a while. Im sick of the dance you have to do to prove your not a stupid uninformed female when it comes to these situations. Anyway, this is a "repair" forum so ...One bad apple doesn't mean the oranges are no good.
There are shops and/or mechanics that are worth their weight in gold. You've just found one that uses sand instead. Don't sell all shops down the river over the actions of one, but beware no matter what! Being informed increases your chances of getting a proper repair.
If you can prove to them that their work was subpar, make them fix it and this time, take a chair right next to it and observe every move without interfering. If they're not comfy with that, they're incompetent to start with and don't deserve to be working on your rig, or anyone else's for that matter.