I have Yukons but in the 4.88 variety. The web site didn't show 4.56 in a reverse cut though. That was at Randy's ring and pinion. You might call them. Someone had posted that they were told something about lockers making a difference, was that you? Any way front and rear with bearings and seals was around 800.00 for the whole job. Almost 20k with no problems. Changed the gear lube last week and it was clean and nothing abnormal on the plug magnet.
That was me. I talked to Yukon and I asked the person if they had 4.56 ring and pinion, and complete 4.56 carriers (pumpkins). When I mentioned I had lockers he said no, then checked with someone else there and confirmed the no. I think others have said they knew of people running the Yukon 4.56's so now I don't know if the guy meant they did not have the locker/carriers complete or if they didn't have the ring and pinion either. He sounded like they did not have either.
The problem is with the reverse cut of the front gear. Older landcruisers that are part time have the same rotation front and rear. Not sure of the early FJ80s which way they are. Honestly for the price of a set of factory gears you can get 4.88s and a nice set of new 315s to go with them :G
From my experience with several european companies I beleive that the gears come from over there. Europeans write their 1s as a upside down cocked V. If you've seen this you'll know what I mean. Both sets that I recieved had serial numbers with this 1 in it. Probably more of a suspicion than anything I guess. I did have some slight harmonic vibrations when they first went in but that has worked it's way out.