Jump seats can be installed in your truck, but I wouldn't consider them safe for on-road driving, especially with a car seat. Look for an original rear bench. It folds up, with the back folding flat onto the seat and then the whole unit liftiing up vertically. You lose some cargo space, but it's the safer way to go for your children.
The bench comes up for sale here from time to time.
1997 P38 4.6
1998 P38 4.0