No no not 20-30k then you'd have pretty much 4 wheel drive... if your bashing then it would be good though. Most ofnas only come with thin grease in all of the diffs and they don't do anything. You should get yourself some diff oil to fill your diffs with and it will drastrically improve your handling. If your running on track a good base setup is 5,000 weight oil front, 7,000 center, and 3,000 rear then change from there according to how you want it to handle.

If your bashing just go heavier with the oils to get more evenly distributed power to the wheels (with lighter oils it will put more power to the wheel with the least grip which isn't good for grass or most other bashing conditions).