Yes you can join Pakistan Armed forces, but you will have to give up your other nationality.
Now i know that USA and Pakistan both allow dual nationalities, but in either case, you have to give up one for joining the armed forces.
Consider your options carefully. Because if you start to serve in Pakistan and give up US nationality, you options might be limited. I mean, what if you don't want to progress further in the Pakistan armed forces? Then what?
I know there are some countries, like UK/Britain, that allow you to reclaim the citizenship back. Check with your state-department or immigration services and see if you can reclaim your citizenship to USA if you leave Pakistan armed forces.
Pakistan Airforce would be good option, if you can qualify for the GDP and fly the skies. But whatever decision you make, stick with it.