Inciser,
If PLB is the only other lower body weight workout that you have then it would be a good sub for LL. Leaner Legs is a very endurance based leg workout that is high rep and it has different moves than PLB, or similar moves with a lot of low ends thrown in (this was the workout where Cathe "discovered" the low end}( ), so if you have any other lower body workouts let us know and we might suggest another substitution. Otherwise, I would use PLB and lighten up the weights, but do all the standing work twice through, skipping the floorwork.
HTH,
Mattea