trans problem

I have a 1991 rodeo my check trans light came on,my fluid is fine but when you go to take off from a corner it is like it is still in drive.Once you hit the gas harder it slames into lo and the takes of fine,when the light comes on.If you stop and shut the truck off,then restart it it goes out and shifts fine till the light comes on again.Please help if anyone has any ideas.THANKS