Needles in gauges just peg out

That's what the limiters look like on cars without the rallye dash (has no oil gauge). Anytime you replace the limiter, I'd recommend using one of the new solid state units as said above.

It sounds like the gauges themselves are OK. Seems like odds would be against having two bad limiters or both gauges being installed incorrectly, but it's possible. A test is in order before spending money on a new limiter.

Try this with the limiter that's not installed in the dash: attach the battery - terminal to the ground terminal, attach the +12 terminal to battery + and measure the voltage at the output terminal. I'm trying to find a diagram to confirm which terminal is which. If the limiter is bad, you'll see either zero or 12 volts at the output. If it's good, the voltage will oscillate between zero and 12 volts. If the limiter tests good, look for the problem elsewhere.

Another possibility is the ground connection for the dash is faulty, possibly the socket the limiter plugs into.

OK- based on what I see in this link:
http://www.allpar.com/history/mopar/electrical2.html

The terminal that connects to the metal shell of the limiter is ground, the middle terminal is +12 and the "angled" terminal is the output.