You'll need to follow the advise above, but you should start running higher octane gas too. At about 75,000 miles, my NA engine started pinging exactly as yours is. I checked timing and replaced dist. cap, rotor, plugs and wires to no avail.

Finally, someone suggested I fill-up with 93 octane gas. Never pinged a ping after that unless I went back to 87.

87 Montero, bought new, by me -
88/89 intercooled Starion Turbo engine-
with awesome audio.