First off, the oil pressure gauge is a 'dummy gauge'. Ford originally had an actual oil pressure gauge, but people kept bringing their trucks in complaining of low oil pressure when it was just fine. So now the oil pressure gauge flies to the center as long as there's at least six (yes, SIX PSI) minimum of oil pressure. There's ways to get around this, but that's beyond what I'm talking about.
On the other hand, the coolant temp gauge is more truthful. I believe there are two water temp sensors for the Ranger, right near the water pump. One is for the ECU and one is for the gauge on the dash. I don't know which is which, but with the vehicle warm, disconnect one of the two. If the gauge drops or spikes one way or the other but then returns normal when said sensor is plugged back in, you've found your diamond.
It's possible the cluster has gone bad, it's possible that the sensors are going bad, and it's also possible that the wiring itself could be damaged. If I were to replace anything, though, it would start with both the water temp sensor (for gauge) and the oil pressure sensor for the gauge as well. I don't *think* they're too expensive, but I've never priced 'em.
One thing you could try before replacing them though, is whipping out some contact cleaner and giving both the sensor and it's connections a good soaking with that. WD40 makes a contact cleaner that's pretty good. It can be found in music shops, electronics stores, hardware stores, heck even Walmart has it. It could be just dirt in the connectors.
It's a rare, rare, rare occasion that the cluster itself is the culprit.