It's hip flexibility that's the real key here. If your hips are inflexible you can't lift with your legs because you hinge by your spine, you need to keep the spine in a neutral position the whole time and let your hips bend and then straighten. Think of getting up from a straight sitting position from a chair; your back should stay in the same position the whole time and your butt should contract to straighten your legs out along the hip joint.
If the object can't be pulled close enough to do a proper squat you need to hinge your hip more into a deadlift position, where your back stays in the same shape and your hip hinges further back so that the object is directly below your chest and then rise back up using your butt again to straighten instead of lift.
If your back rounds you are putting shearing forces on it, and using weak stabilizers to make up for stiff primary muscles.
If you have stiff hips the way to fix them is to find an object too heavy to move, like a car or metal rail. Put your legs slightly wider then your shoulders with your feet turned outwards from your body so they point back at your center line. Hold the unmoving object with your hands and sink down into a sitting position with your back "neutral" IE shoulders pulled back and spine upright like sitting with a posh posture. Hold the object so you don't fall over backwards, and position your weight onto your heels. Hold this for progressively longer every day, a couple minutes a day should be the goal.