To the best of my knowledge on Rangers, the standard transmission on the 2.3L is at least compatible with the 3.0 Vulcan V6, which is the smaller V6 that was offered in those years. That transmission was the standard transmission offering from 1988 to 2011. You'll gain around 40 horsepower with this swap, up to 140 horsepower. I'm not sure if it will take the bigger 4.0L Cologne V6. If it hooks up, it may. That engine puts out closer to 200. The automatic transmission for both the 2.3L and the 3.0L was the 4R44E and its non-electronically controlled predecessor.The 4.0 got the non-electronic 4R55E, which is a heavier duty model of the same transmission because it was a bigger engine. I would assume the standard transmission for that engine is beefed up a bit as well.

You should notice the power difference in a 2WD Ranger going from the 2.3L to the Vulcan. I have a 4×4 1999 Ranger that has the Vulcan. With the 4.10 rear end and 4-speed auto, it's underpowered. The engine was originally designed for the 1st generation Taurus. As a result, the torque and power bands are in the high RPM range. If you do the swap, you want to make sure you get the computer from the donor truck and make sure it's also a standard. 

Swapping a motor out on a car like that is not worth it. The V6 engine has more horsepower and torque and you would have to make sure that the transmission can handle all that. If the vehicle doesn't have any problems right now leave it alone.

See what is the correct manual for the 6 cylinder Ranger.  I doubt the trans for the 4 cylinder can handle the torque of the 6 cylinder.