engine trouble

01 v6 prerunner tacoma double cab automatic. everytime i pick up speed, the entire engine acts like it about to fall out. i feels like the engine is misfiring. engine light comes on and says that clinder 3 is misfiring. it acts fine if i put the tranny in park. and rev it. i replaced the spark plugs and it ran great for a trip a hour away and on my way back about 20 minutes from my house it started it again, and does it everytime now. the the truck its self has 175000 miles.I love the truck but i don't know if i love it enough to put in a brand new engine, with that many miles. if you have anything i should try let me know! knightriderbrandon-spam@hotmail.com or on here.

 

maybe check your vacuum lines... there are only so many places really to check.
Fuel and delivery
Spark and delivery

Beyond that its tricky. If you are having it during an acceleration, may be part of something else. Is it Automatic? If not then disregard vacuum