I had my vehicle in for normal service and the tech noticed that the rear diff was lower than normal on fluid. They topped it off,but also informed me that the service computer said that the fluid should be changed after the first 5000 miles( I am at 14,000). They use synthetics at $16/qt. I didn't have it done yet. It dawned on me this may be the reason the posi felt like it locked up going around in a parking lot last month. Anyone change theirs yet?