"Can I reduce it by rotating my tires more often? Are there any downsides other than tire wear? It looks like that's about it, to me."
Sure can, but it'll still be wearing them out more evenly. I think it's more of a driving style/habit thing... I DD mine 350 miles a week with a spool for 1+ years in a 5spd MJ and 7 months in an auto XJ... If one takes LOTS of sharp, slow turns on paved roads, I'd think the rubber would wear off sooner than one who does lots of straight line or highway driving...mileage, temp & pressure has some to do with it as well, but mainly the frequency of sharp paved turns.
I dunno how a NP242 would react in full-time mode with a spool or a locker.
I'm pretty sure Jegs carrys both full & mini's for the 44. I did an (steel) full since I needed a low-gear carrier, and would have been more $ to buy a Lock-rite etc...and a new hollow open carrier. (I didn't have time to mess with hunting up used stuff) I got mine from Desert Fabrication in Apache Junction AZ, but dunno if they still are in biz.
At $200 for the spool vs $450-+ for a Detroit, your piggy bank is awful close to a set of Moser or Dutchman alloy shafts...