Your temp sending unit on the engine might be bad or hav a loose connection, same with the oil pressure, one wire goes to both sensors. THe oil press is close to the oil filter and the temp sensor I'm not sure on your 94, but check to make sure the connection is clean and the wire clip is on tight.
That might help with the fluxtuations in the gauges, but those dash clusters are known for going out and are abundently cheap, $50 or so used maybe less.
Cant believe you still havnt located the clunking.... has to be something back there loose

Do you knowtice it more when turning a certain way, or going up or down hill?
Other than the struts, strut towers, and suspension would only be the fuel tank and the exhaust.
Oh ya, do you hav a plastic boot covering the top of the rear struts? it is attached to the top and protect the strut shaft from dirt and stufff.
Let me know how it turns out!