The 1990 built truck has year specific engine peripherals and ECU so yes you will need to source another 90' ECU. Before you do though, the ECU's are relatively easy to fix - the common primary fault is due to failed capacitors. If the board hasn't sustained damage you could salvage it. Tell tale signs of bad capacitors are obvious leaks and swelling. If the board has been eaten by corrosion a repair may not work.