After my Merrell MOAB 2 mids started leaking, I got a new pair of Salomon Quest 4D IIs. They're comfortable and more supportive around my ankles, and hopefully more waterproof as well.

I took it out hiking with a 35lb pack for 5 miles, and started feeling some aches/tension on my shins. Are these being caused by heavier boots than I'm used to? Or is it just a matter of break-in? Thoughts?

Impact especially under an extra load. Somehow optimal conditioning prevents this for me to some extent, but you manage this through reduced activity. Can be associated with fractures but not in my case.

Maybe that's because shin splints are caused by lack of conditioning.

Didn't have this issue with the Merrell boots last month when I went 35 miles up/down passes with them carrying 40lbs. Only traded out the Merrells because I don't trust the waterproofing on them.

Never got shin splints from rucking in the army. I think it's more not being conditioned for the ruck in general.
Try literally putting one foot directly in front of the other in a straight line as you step. That always helped me go faster and not hurt for the most part  

It's possible. I have a pair of heavy rubber boots that I wear for deer hunting when it's wet. If I walk too far I get woken up with groin muscle cramps during the night. It is so bad I hesitate to wear them.

Shin splints can be from a variety of causes. Lack of conditioning can be one but the reason the muscles in the sho aren't conditioned is likely from having light shoes and switching to heavier ones. Stretch, ice, and work it out. If it gets worse over the next few weeks then it could be another cause

I would think so, when I played basketball in highschool, heavy shoes and a lack of condition would give me shin issues.

This...there could be several reasons.  I've had a career of hard running, ruck marching (and ruck-runs) and I've seen several guys get shin splints.  Bad arches, weight (either overweight or heavy pack weights), long-distance pounding on hardball-roads with bad form/technique, poor conditioning, etc.  I don't remember heavy boots being the sole cause, but they can exacerbate all the other factors and can contribute to either speeding up shin splints or making recovery more difficult.  Other than staying off your feet and doing low-impact conditioning, lighter footwear, and better technique can definitely help recover from shin splints.

ROCK6