If youre feeling low back pain after squatting its likely youre not "tight" enough, regardless of the forward lean that will bother your low back. If you have access to some lifting shoes those would be a good investment, if not some 2&1/2 lb plates under the heels can help, but I would only do this with the lower bar position, with your knee history it may bother you with a higher bar position. Spread the chest HARD, push the knees out, that should help you keep the tightness throughout the lift.