Sometimes the "pinging" that these engines have at high mileage is caused by the wear on the stock rocker arms, and if the rocker arm wear gets bad enough, it can affect the lifters, requiring replacement of both:
http://www.explorer4x4.com/lifters/lifters.htm
Actual engine knock, like hard, violent knock under hard acceleration, sometimes is the fuel injectors. Years ago I had consistently put in cheap gas back when Sam's/Wallyworld was selling the stuff without enough detergent in it, and it apaprently eventually did a number on the injectors, to the point that when I hit the gas, the knock felt like a giant hammer was hitting the engine block from the passenger side with each piston stroke, hard enough to shake the entire vehicle. The fix was a few bottles of STP fuel injector cleaner over the next few fillups, followed by consistent use of Chevron with Techron over the next several weeks/months. The knock subsided and eventually disappeared altogether. These days I only use top tier gas(
http://www.toptiergas.com/licensedbrands/ ) and I haven't had a similar issue since.
I'd say it's definitely worth a shot using some brand-name top tier 87 octane fuel over no-name high-octane. It may not fix it quick, but if that is the issue, you'll be on the way to solving it rather than perpetuating the problem by using the fuel that's causing it in the first place.