You'll need to verify the following:
* Axle Width: Measure the width of both rear ends. Even a small difference can cause problems.
* Bolt Pattern: Confirm the lug bolt pattern (number of lugs and circle diameter) matches.
* Spring Perch Location: The location of the spring perches on the axle housing must be compatible with the 1999 Silverado's leaf springs.
* Brake Components: The brake components (drums or rotors, calipers) may not be interchangeable.
* Differential Gear Ratio: Consider the gear ratio. A mismatch could lead to drivetrain issues.
In short, while it *might* fit, it's highly recommended to compare these dimensions directly between the two rear ends before attempting installation. Failing to do so could lead to significant mechanical problems.