I just upgraded to 5.29's on my rig. Everything was bought used and I don't know the history of the diffentials, other than the rear is a 4 cylinder 8 inch with Richmond gears according to the markings. After installing a previously owned and what looked light used no-slip locker, the pattern was shallow indicating not enough shims for the pinion. My IFS 5.29 front is even more of a mystery as I bought it from florida and had it shipped but I could not pass up the price. It obviously had some use but according to the previous owner had been sitting for some time.
I had the rear rebuilt professionally (pinion depth corrected) and had them look over the front as well. They said I should be good.
So do I need to follow the break-in procedure for new gears? I searched but could not find any info here on the site. I already put my $$ redline gear oil in the rear but the front is still empty. I would rather not have to waste the gear oil if I can avoid it.

I did take the truck for a brief spin with just the 5.29's in the rear, WOW. Big difference from my stock 4.30s.

 

All they changed was the pinion shim? Then the rest of the parts are used/broken-in. If there's new parts other than that then yes, break-in accordingly.

 

Yes, all they changed was the pinion shim to get the proper depth and pattern. I did check the backlash before and after and it is in spec.

I suppose I could compromise, drive it normally then drain some of the gear oil after 500 miles and check for anything out of the ordinary.

 

since it's now going to wear differently, I'd say play it safe and just go easy for 500 miles then change the fluid

 

I agree with The MAN even though there used and "already broken in" even if they were set up properly both times they wee installed there are variable tolerances that will be minute but present and since they weren't installed properly the first time that means there definitely will be some brake in wear so do a brake in type procedure would probably be in your best interest, and i dont know about you but would let me sleep better at night.