Red Line Synthetic Oil. 75W90 GL-5 Gear Oil
I've been running Redline 75w90 in my diffs since buying the truck. Over 337,500 miles on it right now and zero issues. Redline also lists the same oil for the transfer case, so I would recommend you do all 3 at the same time to save some time and effort over the long run.
I also use 30k miles as the change interval for all 3.
New washers for all the drain plugs -
Amazon.com
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2003 4Runner Limited 4wd V8 - Build thread on Tacoma World
FJ tcase swap, VVT intake swap, Solid Offroad motor mounts, Doug Thorley y-pipe, Bold Performance cat-back, ADS shocks F&R, Metal Tech LTHD springs rear, 1" body lift, 285/75/17 Toyo R/T Trails on Sequoia rims, Coastal front bumper, CAD rear bumper