1-Do I need to change the whole axle or just the gears? If I do the whole axle how hard is it to convert the donuts to disk?

2-What are the best donor cars for the 3.73?

3- What's the average price to buy the axle?

 

You can change the whole axle or the gears only, your choice

for the disc swap read https://www.thirdgen.org/forums/brak...ferential.html

donar cars I believe the only axles that will fit with no modification is 3rd or 4 gen rears (and 4th gen you need to check tire fitment)