Front squats require a greater amount of core strength and stability which is generally why your poundage on front squats will be lower.
If you are avoiding back squats for the sole reason of potential shoulder stiffness and would still like the muscle stimulation in your glutes/hamstrings as a primary focus as opposed to wanting to build the quad as a whole I'd recommend incline leg press. Place your feet slightly wider than shoulder width and place them higher than the horizontal midpoint of the foot platform. Placing your feet higher above that midpoint will cause more glute stimulation then placing your feet at/below the midpoint. However, placing them higher and higher will not result in increased stimulation.
If you are doing squats with the intention of building your vastus muscles I'd move more towards hack squats or incline leg press with feet slightly below the midpoint mentioned above.