80,000 mile started to run ruff I changed plugs and wires no change. Check engine light flashes now, Auto store ran code stated it says too lean rear bank poss. vacumn leak. Any suggestions

 

check all the vacuum lines and make sure none are disconnected, broken, cracked, or otherwise allowing air into the system. Also check for cracks in the intake manifold. You may also have an oxygen sensor going bad on you, but not bad enough to throw a code about it. If you have the plastic intake manifold, I'd be leaning towards a crack in it over anything else.

 

Slymer nailed this one i'll bet.
Most common issue on a Ford product is bad rubber vacuum elbows/lines.

Pop the hood and following the hissing noise.
NO air should be allowed into the system after the MAF,if air is coming in anywhere (including exhaust manifold leaks) it is an intake leak and unmetered air is either going into the engine or across the 02 sensor.