I can't seem to find any vacuum leaks anywhere. Any suggestions?

 

Hi, and welcome to FTE!

It seem most times your situation is a result of vacuum leaks or clogged EGR ports.
Being a '98 either could be the case (due to age and mileage).

You can use "search" up top to search this forum for older posts and threads.
Here are a few that might help.

https://www.ford-trucks.com/forums/9...rror-code.html

https://www.ford-trucks.com/forums/8...-too-lean.html

Ford P0171 & P0174 Lean Codes

 

Check the vac hoses at the rear of the upper intake manifold near the firewall, they have little rubber booties that crack and split, giving you a lean condition. I wish I had found this before I went off on a wild goose chase on my 98 4.2. These actuate the IMRC valves and they are only a couple bucks. I recommend replacing them both, it should solve your problem. Funny thing, about a year later I got the same codes again and this time it turned out to be a similar cracked vacuum hose booty in the front of the upper intake.