aquito, for the TC4, I would just go buy a locked diff, also known as a spool. You can get them from Speedtech RC. That way, you won't have to rebuild your ball diff if you don't like the results. Also you can just tighten it up a bit more, like tighten it til it's pretty snug, then back off a quarter of a turn.

Z Fanatic, since you have gear differentials, you'll have to lock it using the now infamous TT-01 technique. Of if you still want to maintain the differential action, you can repack it with a thicker grease.