Two options.
To get a 4.10 gear, low dollar swap you can swap the pumpkin out of a 12 bolt 3/4 ton. Jim Carter's old chevy trucks website has a tech tip on this one. You can also get a 3.90 gear set from Randy's Ring & Pinnion to go in the newer pumpkin, but the cost is high.
The other is to swap with a late model 14 bolt full floater differential. They come with 4.54, 4.10, 3.73 gears. You will need to find one the right width on the hub mount surface, and get some late model wheels with a 5 to 5-1/2" setback, as these diffs are around 4" wider than your AD. Most any pickup junkyard will have a selection of the differentials, the wheels are the tough find.
