Year–to-Year Changes:
Lots more info here: http://www.edmunds.com/toyota/tacoma/history.html
1996 Toyota Tacoma: For '96, Toyota's Tacoma pickup is provided with a new off-road package that includes a locking rear differential. The new unit is available only on regular-cab 4WD models with a manual transmission. On 2WD models it includes a rear differential locking system, 31-inch tires on alloy wheels, and a tachometer. On 4WD models, Toyota charges a little more, but adds the the following options: bucket seats, a center console, and 4WDemand, the more sophisticated of Toyota's two 4-wheel-drive systems.
1997 Toyota Tacoma: For 1997, 2WD models get a new grille and flush headlights. Also, the base 2.4-liter engine gains a new ignition system and a more sophisticated sequential multipoint fuel-injection system.
1998 Toyota Tacoma: This is the year Tacoma received its first cosmetic face lift. Tacoma finally gets a passenger-side airbag, and the SR5 models get a new name--Limited. Prerunner models started in 98. They share the same front and rear suspension parts and geometry, frame, styling and engine options as the 4x4 Tacomas of this year range. Prerunner Tacomas also had the option of the TRD Off-Road package which (like the 4x4) got a rear differential locker if it was a V6 model (among other suspension variations). One downside is there was not option of a 5-speed transmission. The V6 Tacoma automatic transmissions were an A340E. Because of the similarities in the Prerunner and 4x4 models, it is possible to use stock parts to convert a Prerunner to a 4x4.
prerunner to 4x4 conversion - Tacoma World Forums
1999 Toyota Tacoma: Seatbelt pretensioners topped the short list of changes for '99.
2000 Toyota Tacoma: Intermittent windshield wipers became standard, and models with ABS now had daytime running lights. Four-wheel-drive trucks got numerically higher axle ratios for quicker takeoffs.
2001 Toyota Tacoma: In 2001 Tacoma’s received another front facelift, New for 2001 were 4-door Double Cabs with four front-opening doors and a 60/40 folding rear bench seat. All models got a restyled front end with a raised hood and prominent grille. Also added for 2001 was a new S-Runner model--a manual-transmission V6 PreRunner Xtracab with stiffer springs, premium shock absorbers, color-keyed exterior, and special interior appointments.
2002 Toyota Tacoma: There were no significant changes for 2002.
2003 Toyota Tacoma: Antilock brakes were made standard on all models for 2003.
2004 Toyota Tacoma: The Tacoma loses its optional flare-fender StepSide cargo box in 2004.
In 2003-2004 Toyota started using ETCS-I (Electronic Throttle Control Sytem - Intelligent) this is when the throttle bodys are electronically controlled, but also had a cable for back up in case the electronic system failed.
More info on ETCS here:
http://www.toyota.com/safety/electronic-throttle-control-system/