No, a 1999 Chevy Tahoe transmission will likely
not be a direct fit in a 2003 Avalanche. While both are GM vehicles and share some similarities, there were changes made to the transmissions and related components between the 1999 and 2003 model years. Even if the transmission itself *seemed* compatible, the bellhousing (where it bolts to the engine), torque converter, computer interface, and other parts would probably be different and incompatible, preventing a successful swap.
You'd need to find a transmission specifically for a 2003 Avalanche or a very close model year to ensure compatibility. Consulting a parts catalog or a reputable transmission specialist is highly recommended before attempting such a repair.
