I've looked all around and can't find an answer to what I'm questioning.
I ordered the Sandking XL. Had it delivered to my garage. And fully modded it. If that truck explodes or somehow gets lost, will it be back in my garage? Or do I need to purchase all that stuff again?
If it gets lost, as mentioned above, it should show up at the Car Impound Lot (it has been known for a while now that cars may disappear randomly, and Rockstar commented about it here), but if I'm not mistaken, if your car explodes, it's gone and does not show up at your garage.
However, if you were to purchase vehicles in GTA Online, I heard that your lost or blown up car will respawn in your garage if it's insured.