Transmission Problems

I have an 86 s10 blazer with a 4 speed automatic. The problem I'm having is that it won't drive in over drive it just revs, when I put it in drive it drives fine. When I'm driving on the highway I can manually shift it into over drive and it drives fine. But when flooring it to pass a car, it kicks down,slips and the engine just revs. Can some one help me with this?

How is the fluid level and fluid condition and filter?
Have you removed the pan to check filter and for trash/metal in the pan?
Has anyone used silicone sealer on pan gasket/
Is the TV cable adjusted right?
On a normal take off in OD at what MPH does it up shift?
1-2 and 2-3 and 3-4 and does TCC lockup work?

If it slips in 3rd after a downshift, start looking at the 3-4 clutch, or general condition like low oil pressure from a weak pump or governor problem.