Will a 1999 Chevy Tahoe 4x4 transmission fit into 2003 Tahoe?

While both are Tahoes and share some similarities, a direct swap of a 1999 Chevy Tahoe 4x4 transmission into a 2003 Tahoe is not guaranteed to work without modifications or issues. The transmissions may have different bolt patterns, input/output shaft lengths, electronic controls (especially if one is an electronically controlled transmission and the other is cable-controlled), and potentially different transfer case interfaces.

Even if the physical dimensions seem similar, the internal components and computer integration could cause problems. You'd likely need to check the specific transmission codes (found on a tag on the transmission itself) for both vehicles to see if they're compatible. Even then, professional assessment is recommended.

In short, don't assume compatibility. A mechanic specializing in transmissions should be consulted before attempting this swap to avoid costly mistakes and potential damage.