Zeelott

Hello all when I hit the gas only the back tires are spinning, how do I tell if my front diff is bad. I took it apart and looked at the gears inside and there were no shavings or broken teeth that I saw. Could the gears just be rolled and worn down and not making contact enough? I spun the drive cup while looking in the diff and the gears were all moving. I already ordered the inside gears and will replace them, I just wanted to check if anyone has any info on this. Thank you.