The 4d55/56 engine has been in production for 34 years and is still in production and being used in new vehicles around the world. Parts are easy to come by if you know where to look. I recently purchased a new crate motor for a restoration project. They are great little motors.

I also think the 2.6 is a great little motor and you are not giving it enough credit. The Mitsubishi Starion equipped with the 2.6 TSI engine was faster than a Porcshe 944.The Starion is a highly respected sportscar. My guess is your 2.6 is not running correctly. The 2.6 should have no issues moving your little truck. I had an 85 Ram 50 and it had no issues pulling steep mountain passes at 70mph.