Is Barnie on any glucosamine/chondrotin supplements? You should probably put his on G/C--Cosequin DS is very good (a bit expensive though).
There are many things that can cause a dog to limp in the rear legs--hip dysplasia is one that is quite likely. Many Labs are prone to developing it. Do you know if Barnie's parents have OFA clearances for thier hips? Even if parents have clearances, him being overweight can contribute to joint detirioration.
I think ACL is also quite common in overweight Labs. I hope your vet can determine the cause of Barnie's problems. You might consider taking him to an orthopaedic vet, too--regular vets are just not skilled enough when it comes to bones and joints.
To help him lose weight but protect the joints, swimming works great. Also, many manufacturer suggested amounts to feed (found on the food bag) are extremely high. Try to cut down on Barnie's kibble and supplement with green beans instead. Also, watch those treats--they add up.
Let us know what the vet says. And hope Barnie just has a sprain.
When my mom's lab was 7 years old, she found out that he had arthritis in his hips. He was overweight too. My mom went to the vet, got some medication. She walked him 2 times a day, reduced his food. A few months after, he was in good shape. So she took him off the medication. Everything was good until this year, now Lumbee (lab) is almost 14 years old. She brought him to vet this year, and hes back on medication. (glucosamine) It seems to help him. He just cant walk for a long time or swim. I hope everything goes well at the vet.