If you want something a little higher end than the Polks, theres also these:
http://www.infinityspeakers.com/est...earchMode=regularProductOnly&sCatId=cat740046
I have those in my dash and they sound phenomenal. Replaced 230$ MSRP Rockford Fosgate 4x6s (
http://www.rockfordfosgate.com/products/details/t1462) with these and these blow the RF's out of the water.
And I'm a big Rockford fan, so thats saying something. The 4x6's blew in about 7 months, I wasn't impressed considering every other Rockford product I've owned has been nothing short of amazing.
Now, that being said, you are never going to get much of any low end out of a 4" speaker.
If you REALLY want to improve the sound in the cab of one of these trucks, invest in a nice subwoofer setup and/or maybe even custom kick panels with 6.5's in them.
Kick panels are incredible, and in my opinion, one of the only ways to really get GOOD quality sound in these trucks. They can fit up to a 6.5" speaker and a 1" tweeter. These are mine:
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http://www.qlogicenclosures.com/q-products/q-forms/
Now, the other issue is that, replacing the rear speakers in these trucks is nearly impossible without breaking the panels that the speakers are hiding behind.
I'm seconding what df2x4 said. Run from that deck fast. Brand names are good when it comes to decks. Alpine, Pioneer, Sony, Kenwood, JVC (now part of Kenwood), and a couple of other brands are ALL good deck makers. Don't bother getting a fancy flipout deck, they really aren't all that fancy. What you gain in cool looks, you lose in functionality. If you want a big touchscreen, I'd recommend a double-din, but if you're in an 88-94, that won't be possible without pretty deep pockets. (Around 6-800$ to get into, including deck and a custom made Double-DIN adaptor from Driven Nashville)
So that leaves us some Single-DIN options. For 230$ you can get a damn nice Single-DIN with lots of sound quality settings/adjustments.
http://www.alpine-usa.com/product/view/ute-52bt
http://www.alpine-usa.com/product/view/cde-153bt
Both of those have a 24bit D/A converter, a 9band Equalizer (you want atleast a 5band EQ in my opinion) and other nice features.