There are many different reasons why your engine may be pinging, but it takes experience, understanding and good troubleshooting skills to narrow things down.
Did you buy this vehicle new?
Has it always pinged on 87 octane up hills or did the pinging suddenly start or gradually start happening?
If the engine computer management system has been reprogrammed for more performance from a Super Chips Micro Tuner programmer or something similar, then you'll need higher octane fuel to satisfy the engine's requirements.
If the ECM has never been reprogrammed with a performance tuner and no chips have been installed, then it could be as simple as a dirty mass air flow sensor.
I'm not familiar with the exact details of 5.0 used in the 1996 Explorer, but finding the cause should not be terribly difficult for a smart tech with experience. Maybe the distributor advance isn't what it's supposed to be (bad advance can, weak springs, etc) or perhaps there's a vacuum / intake leak or perhaps the timing is too far advanced (find out where it’s at and try retarding it a degree or two), etc. It could also be a weak coil producing too weak of spark or maybe there’s not enough EGR.
There’s also the PCV to think about and it too can be the root cause of pinging under load, although most people will overlook it because they don’t see the relationship it might have to the cause of pinging. Underneath the valve cover on some vehicles is a splash guard / baffle around the PCV and this splash guard / baffle has a hole that permits accumulated oil flung on to it to drain back down into the engine. If the hole in the splash guard / baffle clogs up, then the PCV sucks in oil, which of course flows into the intake and then into the combustion chamber where it combines with fuel, thus lowering the fuel’s octane rating to some degree.
Also don't overlook something as simple as the fuel filter, which again is something most people wouldn’t think of when it comes to pinging. If the fuel filter is too restrictive (dirty / partially plugged), then the fuel pressure reaching the engine may be slightly reduced from what it should be and this might first start to show up as a pinging problem when the engine is under heavier loads (too lean of mixture for higher fuel demands).
It’s also possible the pinging is caused by carbon build and maybe FP60 hasn’t cleaned it all up? You could try using a more potent intake / carb cleaner and suck it into the engine through a vacuum line.
There are more reasons as to why your engine might be pinging, but I hope I’ve given you a couple more things to think about. Good luck and I hope the solution is something very simple and inexpensive for you.