91-99 were all DOHC Twin Turbo, AWD, AWS(i think, 91-95 for sure)
91-92 have cast crank shafts that werent too strong and 18 spline transfer cases
93+ have forged crank shafts and 25 spine transfer cases
91-93 have 5sp manuals that are better geared for the 1/4 mile if mild upgrades are done.
94-99 have 6sp manuals that are farily well geared for an 11's car.
All years have weak syncro's in the tranys
94+ Got bigger brakes, slightly different exhaust, 2psi more boost and a revamped look.
91-96 have active aero
In 99 they changed the lifters in the engine to a bigger oil boar and got rid of the lifter tick problem as well and changing the front bumper, headlights, and the rear spoiler.
Im sure I missed some little stuff