That’s not an engine I’ve had any experience with personally although I did own a Jeep Cherokee Sport with the 4.0 litre in-line. It must be an interesting one, giving you the same cubic capacity as the early V8 Grand Cherokee engine but keeping the in-line formation.
If I understand your post correctly your oil pressure is OK while cruising but drops pressure immediately if you lift your foot. Doesn’t make sense, the only thing that would change might be the crankshaft thrust bearings or rather the position of the crankshaft relative to the thrust bearings. With accelerator pedal down the crankshaft is pushed forward by torque converter reaction but when you lift your foot the pressure is released.
Did you build the bottom end yourself? How sure are you that the crankshaft thrust bearings are installed correctly? Thrust bearings are generally located by a couple of tabs and if they’re not fitted correctly they can start to spin and then they don’t last long. If the thrust bearings fall out of position that would lead to a noticeable loss of oil pressure but not as dramatic as you say.