Jim Weed lists the process for swapping the earlier Dana 18 2.46:1 gearsets into a Dana 20, with help from a Bronco Dana 20 'case. This could be a major PITA for most of us, and you need to have a Dana 18 and a Bronco 20 (which seems a little difficult to come by) lying around for parts..

I came upon this posting on the Off-road.com Scout newsgroup and thought it might come in handy.. I was thinking about doing this, *and* cutting the front output of the Dana 20 off, then bolting a divorce mounted NP205 from a Travelall or Pickup behind i t for 2.46:1 low, 2:1 low, or almost 5:1 low combinations.. even better than blowin' the money on a Dana 300 and the 4-1 gearset for it.

          Dana 300 transfer cases are hard to find and can get pricy. If you
          can't find a reasonably priced unit you might want to get in touch
          with Ed Holbrook at Holbrook Enterprises (503) 655-4747. He converted
          my Dana 20 with gears from a Bronco Dana 20 and a Dana 18 for a low
          range of 2.48/1. The cost was $350.00 plus shipping. The case can
          be sent via UPS and cost me about $25.00 each way. For the money you
          get almost the same gear reduction and a rebuilt transfer case. Mine
          works like a dream.

          Mike