2001 Bravada Hesitates

It runs great until it gets to operating temp then it starts to hesitate when starting out or going up a hill. It gets worse when I pull my trailor. So far I have put in Fuel Pump & Filter, New Plugs, Cap, Rotor, Wires, O2 Sensor, Cleaned the EGR Valve and put a screen under it, and new PVC valve. I have a lawn care business and need my truck daily. If anyone has any other Ideas I would greatly appreciate it. Oh, Yea! I also looked at the exhaust manifold and did not see any cracks that might leak air in and mess with the O2 sensors.

Mileage and what engine?
Any check engine lights?
Has it set any codes?
Have you run a fuel pressure check on it?

Look at the vac lines under hood drivers side and the large vent tube off intake also. If its a sensor hope it throws a SES light so its easy! Oh yea, don't forget the sec air injection that is under the radiator and is vac operated also so pull the skid plate off and check those too. That system also has exhaust check valves and rubber hoses that go but usually get a code.

ahh.. It threw a code today.. Not sure what it is yet.. Won’t know until Monday.. Hopefully that will tell me something.. I will keep you updated..
Engine - 4.3 Liter
Just put a new fuel pump and filter in it...

I do believe the secondary air unit is not working.. (Pump under radiator) Could this cause the hesitation I am experiencing

Shouldn't unless totally separated vac lines - usually only keeps the SES light on with the code, but could affect the O2 sensor readings thus mixture. Causes lots of noise when those rubber hoses burn thru as the check valves rust internally - see "How To/Tech" section.
All Advance and AutoZ stores read codes for free and would guess most other major chains too.