I own a 2007 Nissan Titan 4x4 and a few months ago my rear end went out. I purchased a new rear end, it was installed and now it is not staying in gear. It stays in reverse and will drive in reverse just fine, however it won’t stay in drive and keeps shifting back to neutral. Mechanic is saying I need a whole new transmission but it is not throwing any codes. Could this be a bad rear end that I purchased? The rear end I purchased came out of a 2wd Titan (same year model) but the mechanic said it was fine. I hate to drop a few thousand on a new transmission when it could be something wrong with the rear end that I purchased.