Plugged cat converter? (very common)
Fuel filter plugged? (kinda common on neglected/high mile trucks)
Fuel pump dieing? (kinda common)
E brake stuck on? (stupid question I know, but Ive seen worse)
Need more info like: milage, fairly recent maint done, any SES light, other drivability symptoms besides it wont rev past 2800RPM, etc.