Toyota Repair: 99 4runner fuel system, clogged catalytic converter, mass airflow sensor

Question
I have a 99 4runner 4x4 with 6 cyl engine and approximately 100K miles with no real problems till now.  The Service Engine light came on and after running diagnostics it was found to have a generic "too lean" reading.  I ran it for a while with no noticeable difference in driving until it started becoming very sluggish upon acceleration.  It starts fine and idles fine but when you accellerate it begins to stall out. We have replced the fuel pump, fuel pump pickup, fuel filter, return line pressure regulator, sending unit from the motor to the pump and changed the gasoline out completely and cleaned the tank (No reall issue there upon inspection either!). We've checked vacuum hoses and all fuel lines. The O2 sensor before the catalytic converter was changed after the lights began with no change in condition.  On the return fuel line past the regulator we found approx. 100 psi by placing a gauge on the line itself, which seems to be excessive but I'm not sure of what it should be. New air filter and all is clean and clear from there to the manifold.  Pulled the exhaust off and checked to see if it was a clogged catalytic converter, ran the same with or without the exhaust. It has never smoked or had any real exhaust issues. And I changed the Spark plugs with OEM Denso's that were gapped approximately 1 year ago they are now blackened with carbon buildup (all 6 equally so) and we had all three city mechanics in the Govt. shop scratching their heads over this one...My local Toyota Dealer's shop is useless so I'm looking for advice or suggestions

Answer
About the only thing that hasn't been replaced is the mass airflow sensor located in the intake just behind the air filer, if the code was a PO171 system too lean condition then try replacing the MAF sensor, in about 90% of the 171 code this fixes the lean condition.