Rod knock on an engine with 4000 miles??????

[highperf4x4]

I can tell you real quick how to check the main bearings and rod bearings. If you have a garage pull the front of the truck inside the door. If not, put the passenger side as close to your house or other wall/fence as you can get it and roll the window down. Bring the engine up to a constant 3000 rpm. After a couple of seconds, let up on the accelerator only SLIGHTLY so that it doesn't drop more than a few hundred rpms. Do this several times. If it's a rod or main it will make a dinstinct clanking noise each time you do it. A loose torque converter will also do the same thing however, you said it does it under a load and a torque converter usually will not so I'd rule that out for now.

With the engine idling, you can also take a wooden handle (like a broom handle minus the broom) and put one end against the engine block and wrap your hand around the other end (flush with the end) and then lay your ear on it like listening through a wall. If you have something knocking you will hear it through the handle and this saves you having to buy anything to listen with.

Not to throw a damper on anything but if your oil pressure is varying from 30lbs to 70lbs, you have a main bearing problem. It could be a gauge malfunction though so who knows.