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Deak, we really need a lot more information than what you have here ie year, model etc.
First thing to do if not already done is to have a service done on the tranny. If it has been a while, it should be done. What condition is the fluid in, does it smell burned?
A little while ago I had a hard shift problem with my F250 that was fixed just by disconnecting the battery for a few minutes and then reconnecting it. Something to try anyway, good luck.
It's a 1996 Explorer Sport, 4.0. I'm not sure which trans. I have had the battery right out of it. We changed the fluid and filter etc. There was a torn gasket but no success. Engine light comes on. You can manually down shift. There dosen't seem to be any slipping.