Um, leave the tranny in reverse?....
KIDDING!! Not sure what you mean by "reverse" flush. It doesn't matter which way the coolant flows when you flush it out, as long as you get all the old stuff out. There's no advantage to making the water flow in the opposite direction to what the water pump pumps. And if you use any method that involves running the engine, you'll never get the flow to oppose the water pump.
In any event, this: Gooch's Radiator/Engine Backflush Procedure and Coolant Information - Ford Truck Enthusiasts Forums
is probably the most-followed procedure. It's a little overkill to me, in terms of how many "repetitions". Important take-aways are:
* Use the tee and an extra piece of heater hose, to preserve your existing hose
* Know where both block drains and the radiator drain are
* Do the flush with the thermostat removed. Good time to replace the thermostat anyway
* Finish with ONLY DISTILLED water and at least 4 gallons of undiluted anti-freeze (our generation of truck should NOT use extended life anti-freeze)
* MUST have sufficient SCA - either added (DCA4, etc), included in the anti-freeze (FleetCharge), or in a pre-charged coolant filter
If you think you've got a restriction, in either the radiator or the heater core, you can reverse flush them individually, but I would do that separately, before the complete flush/refill, and isolate the restricted component from the rest of the system.