Will a rear axle from 1994 z28 Camaro fit in 1990 rs Camaro?

No, a rear axle from a 1994 Z28 Camaro will not fit a 1990 RS Camaro without significant modifications, if at all. The 1990 RS likely has a 7.5-inch 10-bolt rear axle, while the 1994 Z28 likely has a stronger 8.5-inch 10-bolt (or possibly a 12-bolt depending on options).

Even if both were 10-bolts, the differences in the mounting points, spring perches, and overall width are likely incompatible. The brake and suspension components would also need to be swapped and may not be interchangeable.

In short, it's a much more complex undertaking than simply swapping the axle. It's more realistic to rebuild or replace the existing rear axle in the 1990 RS.