Davey, I'm not an expert, but the ideas referenced here by Don, JPM, & oldtimer have benefitted me on a personal level. I have semi-recurring lumbro-sacral issues, and exercises in which the weight is held below the body's center of gravity are much more forgiving on the lower back region.
There's no guarantee that this type of training will not yield problems for you; a disc issue is serious, and has the potential to worsen.
But - if you decide to try the principles mentioned above - you may also wish to experiment with Jefferson squats and old-fashioned barbell hack squats (not the machine).
Also, I knew two national-level female bodybuilders who had outstanding leg/glute development. Both used step-ups as a mainstay in their leg training. Some gyms offer step-up machines consisting of an adjustable height platform with two cable weights to use in lieu of dumbbells, which is ideal for performing drop sets - although, if using db's, you can always just ditch them at the end and rep out with body weight.
Lunges are also intense. Personally, I've found the barbell version to be more challenging since more work is required to balance the bar, which may recruit more fibers. But, the db version is much safer, and will also build your hand and forearm muscles as farmer's walks do for a lot of guys.
Whatever you do, do it safely, friend!!