This is the first time I have heard this.
I do not remember my crew cab, full 4-doors (I don't know hat you mean by extra cab) factory speakers being quiet in the rear when they were stock.
Are the rear speakers still quiet when you sit in the rear, or is it only quiet because you're up front?
I believe the factory speakers have identical output at each corner in my crew cab. Each of them are like 8w max and are the same size. Unless your radio is failing and not outputting enough power to the rear channel.
A fix, I can't say there is one since I haven't seen this in my own vehicle.
Oh, have the speakers been replaced in the rear? Perhaps a previous owner installed new rear speakers of a high wattage and the factory radio isn't strong enough to make them sound as loud as that speaker is meant to play. Typically, a 200w rms won't run good on an 8w radio, for example.