valve covers are not leaking however in the rear of the engine possibly the intake manifold there is oil leaking down both sides of the engine,I cannot see because motor is tight to fire wall,Im sure the rear main seal is leaking also but oil is coming from top of engine in rear,is the oil sender back there ?,also the front left side looks like oil could be coming from the head gasket,are these motors prone to head gasket oil leaks ?

 

Not sure about the other questions butthe head gaskets will go in these trucks almost definately. Mine went when I had about 60,000 miles. How many miles do you have?

 

How many miles on your 99 D2?
V/C gaskets are a common leak and the valley pan gasket will usually be the oil leaker at the back side of the engine.
Is it dripping on the ground or just seaping, have you ever power scrubbed your engine to get all the oil off and get a better idea what is leaking?

 

it is dripping to the ground and the valley pan gasket would make sense,it has 100k on it and uses/leaks about 1 qt every 3500 miles,when you say head gaskets go are you referring to oil leaks from them or antifreeze into the cylinders ? I am a mechanic and its my friends car,I dont want to do a valley gasket and find out the head gaskets are a problem and the rear main seal also,I would have to really clean it good and look for it leaking ,just trying to find out common problems on this year,I told him to just check the oil every 2 weeks,alot cheaper