Did you give a good look for vacuum leaks?
There are a number of threads here that have a good link for 0174 codes, regarding clogged EGR ports.
I'll bet you end up going in that direction.
If the plugs that you pulled looked good, and you had the problem, and replacing them made no difference as you noted above, I wouldn't go that route, at least not at this time. Those one's in the back are a bit tough to get to.
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BroncoJoe19
2006 Alt Fuel Jeep (electric)
'98 windstar 3.8L
'99 Crown Vic 4.6L
'90 Full Size Bronco 5.0L
I'm not a professional mechanic, take my suggestions with a grain of salt, or a cup of coffee 
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