Four wheel drive help 2000 GMC 2500 6.0

While in four wheel hi or 4 wheel wheel auto going at higher speeds 40 or 45 the transfer case seems to slip into netrual and the engine will rev out while hitting the gas. As soon as I slow to almost a complete stop the transfer case seems to engage again and I can drive. It doesn’t happen while in 2 wheel or at low speeds while in 4 wheel 35 or lower. The truck has 140,xxx miles on it. Any information would be greatly appreciated.