On a 1999 Expedition when trying to pass another car or when going uphill your truck shakes and does not pick up speed what causes this?

One of the 1000 posibilities is an ignition misfire, but will be impossible to diagnose without personally driving the vehicle and doing diagnostic checks. On of the most commn things I see go wrong with Exp'ns, is failed ignition coils. You didn't say whether you had a 4.6 liter(separate coil pack and plug wire system) or a 5.4 liter (COP ignition system), so I don't I dont know what type of ignition system you have. Another of the seemingly endless list of causes that your truck could have is low fuel pressure due to a faulty fuel pump.