Generally I just go to a Ford dealer parts site and look it up there, for example (and this is for a 5.0L V8 EB, drill back to engine choice if you have the 4.0L engine):
In other cases you can go on Rock Auto and search for parts and a Motorcraft might be listed, or for the aftermarket parts, some will also show a cross reference to the Motorcraft part # as "OEM Interchange", for example here's a control arm for your '98:
These are old vehicles, many parts you may need, someone else has already posted about them in a topic here on this forum. I may know some of them, what do you need?
As a last resort (because it is so old and low resolution so a pain to use), you might check the 2nd gen Workshop Manual linked below in my sig, or often you can call your local Ford dealer parts department and they'll look up the part # for you.
As far as diagrams, beyond what's in the Workshop manual, or info in this forum, it may be even better to find a youtube video doing the repair. Chilton's or Haynes manuals can also help in some situations, but many times I find their info to be copy and paste generic info, beyond sometimes supplying torque values for fasteners but I have to admit that for most fasteners, I just wing it as I have a reasonable feel for how tight a certain size fastener into a certain material needs to be. I wouldn't rebuild an engine that way.
