Plenum? (long Shot) My plenum went out i didnt notice just thought my motor got old lost power so on now replaced plenum and clutch fan and drives as good as first day i got it. My second guess would be cat as those things seem most have to be replaced

 

check you upper and lower plenum gaskets. sometimes the bolts will drop out of the bottom of your intake and fall into the valley where your push rods reside. if this has happened you engine can be sucking dirty oily air from the crankcase and trying to burn that. to check if it is this, see if ur truck is smoking blue(sign of burning oil), pull ur plugs and see if they are oily or built up with thick black soot. sometimes if it is leaking really bad u can see oily residue in ur intake manifold when u remove the throttle body(while u have it off, give it a good cleaning). my brother has a 92 dakota with fuel injection and he had lost power all of a sudden and it started to blow blue. I pulled off his throttle body and the whole intake was coated in oil inside. hope this helps.