99 Burb Loss of power/ Studders

Truck studders and has loss of power when
pushing on the gas.  Works ok if flat ground
and I slowly give her gas.  She hates hills and
1500 RPMs/ OD.  She does better in 3rd and
above 2000RPM.  She starts and idles like
new.  

I have given her new wires and plugs, gas
filter, changed both Cats, Intake Manifold
temperature sensor, fuel injection idle control
valve, O2 sensor and throttle position sensor.
Cleand K&N air filter and throttle body.  

My next ghost chase is a vacuum leak, but
not sure how to properly do that.  She
seemed to run fine before I shipped her from
Guam back to the states.