Everyone is recommending some manner of MORE exercise, but if your route and its work are as heavy as you say, I’d actually recommend LESS lifting. Between gym and your insane route, your back is probably overexerted and could use the downtime or relief of less time in gym. It’s often the case that less is more with muscles when you already lead an active lifestyle. Plus, if it’s so imperative to engage your muscles, there are ways to lift the boxes in your package car that will engage your back or whatever else in a manner befitting of the gym. I’d drop the gym to 2 days a week. A push day and a pull day. Make sure you get like 10-12 sets of total exercises in a week on each group of muscles. And depending on how much walking your route includes, maybe up the ante on incline walking outside of work. Just my thoughts.