A 1996 will need expensive reprograming if you change the axle ratio.
The axle and driveshaft for a L99 motor are different than for a LT1. If you have a LT1 you should have the LT1 axle size.
Wagon axles do not have the same mounts as other b body cars. The Fleetwoods have different ABS system.
There are three LT1 axles. B4U with the large brake drums. 9C1 (Narrow) and Impala (wide) with disk brakes.
The information you should read is in Post#3 and Post:4
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Or calling the local used parts places.
It is really hard to total a rear end. Axle shafts, bearings, and gears are still available if you know a reliable axle shop.
4X4 and big truck shops rebuild axles all the time.