One of ours is a pit/lab mix as well- Master of Disaster- "Master" so named for the padlock around his neck when we found him. He's 88 pounds of solid rock.
See the scar inside the left leg? CCL (canine version of the ACL) surgery last August with a 12-week recovery. The ortho vet who did the surgery was pretty reluctant when we first noticed an issue and brought him in. He is so muscular that she had a hard time determining just how much lateral movement (not good) his knee had. Right leg needs to be done, probably late winter/early spring. This is his normal sitting pose now, although he'll occasionally sit upright on his haunches. He can still run and loves to chase squirrels and he's boundary trained, but we leash him for a portion of his daily walks through the woods so he doesn't tear up the other CCL too bad. He's only 1 month since being cleared for full activity from the vet, so we don't want to turn around and put him back in a kennel for another 12 weeks after another surgery.
Just a heads up- it's a fairly common malady among 70-80% of lab and terrier mixes.