When I was in the transition period of changing from Spicer 1760 to SPL I ran into this all the time. The drive shaft people I use do not put a 1760 yoke and a Spl yoke on the other end. I am assuming your auxiliary is too old to be fitted with a SPL yoke, but you can probably get a 1610 yoke to fit your rear. There is nothing wrong with 1610 shafts. I used them on interaxle shafts for over 30 years. I scrapped a bunch of 1610 shafts recently, but I probably still have some Spicer 1610 joints in dusty boxes on the shelf. Unless you log over 100k miles per year like my trucks do, you will probably never notice the difference in life of a 1610 compared to a SPL.