My 2005 4X4 shifts into 5th gear at almost exactly 80km/hr ( 50mph) during normal acceleration. I'm having a problem with the rear end locking for a split second only if I'm on the accelerator while shifting into 5th. Happening every time. Only if I'm on the accelerator though. If I accelerate aggressively up to approx. 90+ (km/hr) and release, the truck will naturally shift into 5th with no issues. Then I can continue accelerating to whatever speed I want. But if I have to gear down from slowing down, I have to do the same and accelerate aggressively to avoid that locking. Thoughts?