Ok, just some rambling thoughts. First, it would be very wise NOT to use the T86. They may shift better and be a bit less noisy than the T90 but they are weaker and parts are hard to find. Worst CJ three speed but slightly better than the T84 of the MB/GPW's.
Realize that the D20 is a 2:1 low range versus the 2.46 of the D18. Aftermarket gears are available for both but that is expensive. The two really big advantages of the D18 are the lower rear driveshaft angle and the overdrive option. As to gears for a 225, 488's and an overdrive work really well. If the 225 is a little built, no problem doing 75 all day. If you do stick with the 18 (and I would build it using the case from your D20) and want an overdrive, the original Warn's are better than the Saturns. The Saturns use a cheesy diecast shifter fork which break on a regular basis (Warn's use bronze and I have never had one of those break).
If I were doing this, the axles would be a front D30 and a rear D44-offset-and keep the D18. You can make a pretty decent D44 from the stock axle by putting a full float kit in it. The next best option is a flanged offset D44 from the 70 1/2-71 CJ's. You get 30 spline axles with either but the latter are hard to find.
1955 CJ 3B, 381cid sbc (441cid all aluminum sbc coming soon), AGE M22W, Teralows, Warn OD, OBA, Premier Power welder, Warn 8274
1968 CJ 5, factory 225" w/ headers/450 cfm Holley, T86AA, Belleview winch, Warn OD
2012 Wrangler Rubicon Unlimited, Warn 9.5 xp-s winch