Why does 1993 Chevy Silverodo K1500 4x4 ext cab slip in overdrive?

Because your transmission is burning out. I have this exact same truck and the OD started to slip about 6 months before it and third gear finally went out completely. As it was slipping it had a bad tendency to "search" for a gear while cruising at 70 mph down the highway, and would constantly go back and forth and would slip into OD when it went. The tranny did have 160,000 miles on it, so I didnt complain when I had it totally rebuilt for $2,000.