That's actually a good point - equipment makes a big difference. PL squats (raw) have always been about 200lbs higher than full squats, though, because of the lessened ROM and wider stance.
To the poster - what form are you using for SLDL? If you're rounding, it's definately due to weak legs/hips.
I'd also wager you have some major flexibility issues if your full squat has you rounding over at the bottom and it's STILL better than your PL squat. I'd take a look at the get your butt in gear article to assess that.