I own a '94 GXE and '00 GLE and found the best place to go for speaker size/depth questions to be
http://www.crutchfield.com. Best bang for the buck amoung factory replacements have, imho, been Kenwoods or Sonys when it comes to proper fit and great sound (though you might want to look into running better wire to aid in allowing as much as possible from your receiver/amp). I'm thinking the max. depth for your vehicle would be in the realm of 4in. for the rear deck and about the same for the doors (being 6 3/4 speakers you don't really have to worry too much about depth since unlike 6 x 9s, they don't have the biggest magnets in the world to contend with). But, when in doubt, check with Crutchfield for mounting depth info.