jabres - I really don't know the codes for the rear axle ratio. Good luck finding a used 3500 SRW with 6.4 and 4.10 rear. I live in Louisiana. When I was looking, there was one 2500 with 6.4 and 4.10 about 20 miles from me. According to the Ram website filtered to show 6.4 and 4.10 by distance to dealer, the next closest one was in Tennessee. I bought the one within 400 miles of me. Point being, the 4.10 is hard to find even trying to buy new from a dealers lot. Most are probably ordered. Good luck.