Why is my transmission slipping? on 1993 Ford Explorer

I have a 1993 Ford Explorer that is having slippage in the transmission. Why?

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2 answers 1 comment
If the transmission is as old as the truck, it might be worn; Has it been serviced any and how long ago, probably needs to see the doctor, clutches are gone.
Low fluid level, worn internal parts.
The transmission fluid smells burnt. We just put a used transmission from a wrecking yard in the Explorer. What do you think about that?