Are you sure the truck wasn't happy. We have a f350 with a 5.4L and it will pull just abut anything. I've towed my old fullsize bronco many times on a equipment trailer.
You have to keep in mind old v8s like the 460 and 454 made gobs of torque low in the rpms. And now modern v8s get compared to desiel engines.
The 5.4 makes power pretty high in the rpms.
That truck has plenty of motor to pull an xj.
