Your expansion tank may have failed or all of the air bled out over the years. To check it, first just take the air cap off. See if it is wet. If it is, then double-check by tapping on the tank. It should sound basically hollow. If it is full of water, either the bladder is shot or the air is gone. Shut off the main water supply before the tank. Open a faucet until the pressure drops. Then, using a tire pressure gauge, check the air pressure in the tank. According to Watts, you should have a 40 pound precharge. That will potentially change depending on the size of the water heater and the final pressure you end up with if you put in a prv.
84# is fairly high, you may want to consider a prv. Putting one in is fairly straightforward if you know how to solder and your shutoff valve works. You might want to consider putting in a T and a pressure gauge while you are doing this since they are fairly inexpensive. If you want to get fancy, put one in before and another one after the prv.
Your expansion tank is too small for this application. Check out this link http://www.watts.com/pro/divisions/w..._DETsizing.htm