I had a random occurrence where I could hear the solenoid click, but the starter would not engage. I, too, had a relatively new starter, so I was fairly sure that it wasn't the starter. Repeated attempts to start the truck would eventually succeed, so I deduce that it was a bad connection...
I had my check engine light coming on and after many attempts to track it down, I finally took it to a former Ford dealership where they read the codes. I replaced the fuel filter and the DPFE. Both of these are relatively easy and cheap to replace. The 1994 4-cylinder now runs like a champ...