When diagnosing an oil leak how can you tell which one of the 3 listed above?

 

All will leak eventually.
If it's coming from the back of the block, then probably the rear main seal. The engine or transmission has to come out to replace it.
The oil pan can be dropped with the engine still in the truck for the cruciform seals and pan gasket.

 

Rear main will be at the rear only, simplest test is get under there clean everything up. Drop the circular inspection plate,if it is covered in oil rear main. BUT before you go there clean it up warm up the engine and drop a second time more oil most likely the main.

 

I'll try to post some pics but there is oil from the starter down to the seam between the engine and transmission. I've pulled the inspection plate before and looked pretty clean

Not sure if oil is running down from up top or leaking around RMS and getting flung around by the flywheel and exiting the hole by the starter?

 

Could be valve covers

 

Or rear of intake manifold too

 

Design of the engine is such that RMS NEVER leaks. Unlike other V8's, it is not under pressure. There is a drain relief in the rear main cap between the rear main bearing and the seal. Let me know if you guys want me to post pictures explaining why it is impossible. It is ALWAYS the crucifix seals (unless it is the valve covers or intake - caused by plugged PCV). The oil level is not high enough for the pan gasket to leak.

BTW, the new crucifix seals absolutely suck. Use liberal amounts of silicone on the surfaces of the rear main cap - do not depend on those crucifix seals to do the sealing.

 

That's interesting, I had a rear main seal replaced by the dealer under warranty. They said it was leaking.

 

I had mine replaced a few months ago it was shot, no leaks since. @Extinct I would like to hear your explanation of why they do not leak.

 

RMS was visibly wet when I first took my 4.6 apart, and no signs of oil from intake dripping down and around it.
I had a steady drip there originally too.
There is a pretty big empty cavity behind the RMS before the rear main cap.
I don't see how the RMS won't leak after it wears out.