The OEM speakers are terrible. Even $20 Wal-Mart Sony speakers will have better sound quality than the factory ones.
The rears may have tweeters, but in reality, they are just basic coaxial speakers with a paper cone in the center to direct some of the highs a bit. There is no separate tweeter up front, and likely isn't in the rears, although I could be mistaken, all the Suburbans I've seen from this vintage in the junkyards have speakers like this:
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What you would want (I'd imagine) are aftermarket speakers with directional (moveable) tweeters. Some companies make some where you can adjust where the tweeter rotates, thus changing the direction the high frequencies play. I've always found it to be a bit unnecessary, but they are out there. Browse
www.crutchfield.com for some, then shop on eBay or Amazon for a better price on them.
Any 2way or 3-way speaker will sound worlds better than the stock ones. I have a stock Delco radio with cassette w/ Graphic Equalizer built in, and have Boston Acoustic speakers in the truck. They sound fantastic through the stock system.