It may be less that you are crossing your leg, but in so doing you are sliding your butt forward, and rounding your spine.
I'm with you that there is more likely two problems going on.
Again, I understand your reticence to stop weightlifting entirely, but would you consider cutting back the weight 20% on squats and deadlifts and really concentrating on perfect form for a week or two to see if it helps?
Even with perfect form there is some strain on the back muscles - that is what makes these exercises so great - they exercise so many areas including the back.
"The Progression of Paraspinal Muscle Recruitment Intensity in Localized and Global Strength Training Exercises Is Not Based on Instability Alone
Juan C. Colado, PhDa, b, Carlos Pablos, PhDb, Ivan Chulvi-Medrano, BScb, Xavier Garcia-Masso, BSca, Jorgez Flandez, BScc, David G. Behm, PhDd,
Volunteers (N=25) without low-back pain.
Subjects performed (1) localized stabilizing exercises (callisthenic exercises with only body weight as resistance): static lumbar extension, stable (on floor) and unstable static unipedal forward flexion, stable dynamic unipedal forward flexion, and unstable supine bridge; and (2) global stabilizing exercises (70% of maximum voluntary isometric contraction [MVIC]): dead lift and lunge.
Normalizing to the MVIC, paraspinal muscles were significantly (P<.05) most active, with mean and peak amplitudes of 88.1% and 113.4% during the dynamic stable dead lift at 70% of MVIC, respectively. The supine bridge on the unstable surface obtained the significantly lowest values of 29.03% and 30.3%, respectively. The other exercises showed intermediate values that ranged from 35.4% to 61.6%.
Findings from this study may be helpful to strength trainers and physical therapists in their choice of exercises for strengthening paraspinal muscles. Our results suggest that in asymptomatic young experienced subjects, the dead lift at 70% of MVIC provides higher levels of mean and peak electromyographic signals than localized stabilizing exercises and other types of global stabilizing exercises.