Have a look at the pin the seat back frame rests on that sticks out of the seat base inner side...there should be a plastic bushing over the metal pin. Most of the bushes crack and fall off over the years...the seat backs get loose and tend to bend towards the centre of the car.
The bushes are available......if you can't find them from suppliers here in Australia, the ycan be brought off Ebay and the like. Ford F100 tail gate bushes are the same size....apparently.
They look like this....
Of course, its not unknown for the frames to be cracked, too......