Crutchfield makes things pretty easy. For our trucks they list only one speaker as an EZ fit into the front doors and that is the Alpine SPS-6939S, a 6"x9" of a depth that clears everything in the door.
They have 25 models of 5 1/4" speaker that will fit in the rear door/panel in quad or club cabs and one of those is another Alpine, model SPS-1329S. This is of the same design as the 6"x9" in the front. Matched sets are nice.
In our '97 I used these. I wanted a simple upgrade of sound and used a pioneer head, a rockford sub in the r/h rear panel and the Alpines. It's a nice cheap good sounding system.
I changed out the speakers first and want to say that those made a substantial improvement in the sound of the stock radio all by themselves. The radio was the cheapy with only a tape head. When I tried the speakers with that radio I almost stopped right there because it sounded very good - much better than the stock speakers do.
I went on and added the pioneer, the sub, and a 12 disk changer, but for someone who just wants to improve their stock radio/stereo sound those alpine speakers at less than $200. for all would be a good way to go.
(by the way - crutchfield had said that some drilling would be required to mount the Alpines in the back. I just used two stock mounting holes instead of four and drilled nothing)
I have the infinity system in our '01 and it's OK, but I've been thinking that maybe a set of those Alpines would help it out of it's muddy sound too.
I don't spend much for sounds in these trucks. There's always music playing anyway, unless the key is off.