The higher impedance of those speakers is cutting your power by at least half coupled with the fact that your factory HU is not rated at very high RMS wattage is also causing problems.

My suggestion to really correct all those problems is get a new Head Unit but I think you probably have a Double DIN OR A 1 1/2 din unit in your truck so you would need a adapter kit which isn't a huge deal. Buy maybe some new front speakers since you can get a DECENT set of coaxials from Circuit City or Best Buy for your needs and then leave it at that. 250-300$ total spent and hell when you sell the vehicle if you do you can easily take it out.

The reason I suggest a nicer slightly newer set of front stage speakers is because your factory ones may not handle the higher RMS from a aftermarket HU.

Otherwise keep what you have and just get some 4 ohm speakers for the rear.

And if you want some help in finding the kit, HU and such just ask. I know of some aftermarket HUs that you might like WITH knobs and such haha.