Q: In regards to shoulder injuries, do front squats place less stress on the shoulder than back squats?
A: This would depend on the shoulder injury in question.
For people with typical impingement problems, the back squat position puts them in the at-risk position (abduction + external rotation). So, for them, the front squat set-up is much better.
For people with acromioclavicular (AC) joint problems, the bar position on the front squat will put some really uncomfortable pressure directly on the AC joint. They'll handle back squatting a bit better.
So, I guess the answer - as always - is "it depends."