2002 Jeep Wrangler low oil pressure and disappearing coolan

Hope someone can answer this definitively as I know these problems are commonplace with the Jeep 4.0 enging and systems:

I have a 2002 Wrangler Sahara and here's my story:

1. For two years, I have been losing coolant. First over the course of months, then full to empty overflow reservoir in one month. No visible leaks ever. I have had the following maintenance donde:
(i) Replaced water pump; (ii) Coolant system flushed; (iii) thermostat replaced; (iv) temperature sending unit replaced. The engine oil does not look like a chocolate milkshake, and there isn't any visible white smoke coming from the tailpipe.

2. Last 3 months, I have zero to low oil pressure upon startup, but the dipstick reads full (oil has always been changed every 3K - 6K miles). Oil pressuer goes to normal as RPMS go up, but never at the typical 40 - 55 lbs it used to be at. Doubt this is a guage problem, but I don't know. There is no engine knock.

I had the engine pressuer tested all day, and the tester never registered any leak. As I said, I have found no visible leaks of any kind in any system on this Jeep after a close inspection.

Is this likely a head/gasket issue? I am at a total loss and would like any ideas before spending the dough to have the engine taken apart only to come to the conclusion that the entire engine is going to go and will need to replaced shortly. I've heard this is a common issue with the 4.0 but I can't seem to find anyone with a definitive answer.

Any ideas?