Why did my transfer case slip into neutral going downhill, but still in gear?

What is your question?

Started yeaterday with full load of cattle on I-64 on North Mth outside Staunton,VA. Had in cruise. Started gaining speed. Had slipped to neutral but was still engaged. Stopped, put in low range,. put back in high, did not engage, put back in low,engaged went 200ft, put back in high, engaged. After got off Mtn was fine. Came back with full load. When started down mtn same thing. Stopped and started 20x. Finally noticed if touched break would stay in gear. Made it home . Any ideas?