* Engine: The transmissions are matched to specific engines. A transmission from a 1991 Chevy 4x4 with a different engine (e.g., a V6 versus a V8) will almost certainly *not* fit a 1989 Chevy 4x4 with a different engine. You need to ensure the engine in both trucks is the same.
* Transfer Case: The transmission and transfer case are connected. The transfer case from the 1991 truck might not be compatible with the 1989 truck's transfer case mounting points, even if the transmissions themselves appear similar.
* Bolt Pattern: While likely similar, there's a slight chance the transmission bellhousing bolt pattern (where it attaches to the engine) might be different.
In short: You need to compare the specific transmission model numbers (found on a tag on the transmission itself) from both trucks to be certain. Even with matching numbers, there's a small chance of minor differences. It's best to consult a repair manual for both years and models to confirm complete compatibility. A parts store or mechanic specializing in older Chevy trucks would be a valuable resource for this kind of verification.
