'92 Blazer with a 4-wheel drive problem

My '92 Blazer is having serious 4-wheel issues. When I'm in overdrive and I accelerate usually up hills, going 90 km/h or over my 4-wheel drive kicks in. There is this really nasty noise and I have to slow down to below 80 km/h in order for it to stop (which it does after a loud thud).

What is the problem? And how can I fix it? Is there an easy way to fix it like disengaging the 4-wheel drive altogether for now?

Thanks