If this is as big as youll go, id recommend the 10% underdrive gears from Cruiser Outfitters ( cruiserteq.com ) for the transfer case ~ this would net you a 4.72 ratio and may be cheaper than opening both diffs.....
You have 4.30 in the diffs now - 98-02 were 4.30. When the 5 speed transmission came out in 2003 - the diff gears changed t o4.10
Recommendation and only option is 4.88s in the diffs.
You can also do 10% underdrive gears inthe Transfer case and do a 3:1 4 Low reduction at the same time. - 4.88s...