I have been getting the P0171 and P0174 codes on a regular basis now myself. Always resetting the CEL, and it will come back within two drives of my truck. I replaced all three sensors 1.5 years ago when I first got both codes. I figured that it wouldn't be a bad idea to replace the O2 sensors, since I bought the '98 Ranger XLT 4.0L with 102K on it. This did not alleviate the problem. Also, I did inspect the PCV rubber hose (seemed tight) and pulled out the PCV valve (which did rattle, but I did not run any solvent through it--definitely should have). This winter, I drove it to WI from CA. It was freaking cold in WI (I am talking single digits), and everyday I drove it, I got both codes. Bought some MAF sensor cleaner, took the MAF sensor out and cleaned it (BTW, I deem you insane for using a toothbrush on it, this is a $130 part to replace and is a delicate part). Back in CA, did not get the code anymore.
So in May, I regularly get the code again. I also have this crazy howling occasionally now from under the hood that goes away when I turn off the AC. I decide to pull the trigger and replace the MAF sensor. Basically threw away $130. Valve cover gaskets look OK and are not leaking to my knowledge. Hose clamps tight. I am going to replace the PCV rubber hose, PCV valve and valve cover grommet outright now. I definitely need to investigate the intake manifold gasket however. It would be awesome to get rid of these freakin' codes!