Explorer Tranmission Trouble

The car starts perfectly well. The transmission will engage in reverse and move. When the truck is put into any other gear it only revs and no movement occurs. You cannot feel it engaging.

Am I doomed to a transmission replacement?

How many miles on what year Explorer?

How long has it had this problem. Have you ever changed the transmission fluid? Is is true you have an automatic transmission?

It is a 1991 Explorer with approx. 178k miles. We have only owned it for six months and have not changed the transmission fluid yet. It is a 4x4 automatic.