Welcome to the forums, Phantom!
If you have an SC300 there are many possibilities for the engine. If you have an SC400 there are far less standard and common possibilities for the V8 engine.
Try Lextreme for things like headers and automatic transmission modifications. oldManTan is right, your options with that drivetrain are very limited unless you spend tremendous money on a fully one-off boosted setup.
Your gearing options are all from the OEM parts bin.
92-97 SC400's came with 3.916:1's
98-00 SC400's came with 3.266:1's (so did the 98+ GS400's)
92-97 SC300 5-speeds came with 4.083:1's (some year GS300's had the same ratio also)
92-00 SC300 Automatics came with 4.272:1's (so did 93-96 MKIV Supra NA's and some GS300's)
There is also the Supra MKIV TT Automatic 3.769:1 ratio.
All of those use the same common 200mm differential architecture. Commonly referred to as the "small diff" in the SC, Supra and GS world.
As for locker differentials we no longer have new TRD 2-way diffs but there are new Kaaz, Cusco and OS Giken clutch-type differentials in 2-way. OS Giken also makes a very expensive 1.5-Way diff.
From the factory only Supra MKIV's and JDM Soarers had compatible LSD's: the very common Torsen T-1 from the Supra MKIV TT Automatic (also unicorn level rarity in some Supra NA's with 4.27 or 4.08 ratios) or the JDM Soarer only Torsen T-2's. All Torsens are 1.5-Way and not classified as "locker" LSDs.
Finding a Torsen isn't easy. Buying an off the shelf 2-Way clutch-type aftermarket LSD from a vendor is.