As long as you have a 28 spline posi then yes you can install 28 spline axles.
Factory 28 spline axles can be found in 1990-1992 third gens only. Aftermarket axles from places like www.tomsdifferentials.com
are around $125 each and will be much better than factory axles. Fourth gen axles are also 28 spline but they're also longer and won't fit into a third gen diff.
28 spline axles are a slightly larger diameter than 26 spline so they're stronger.