I will try my best to help you with your problem, but of course without the car in front of me, it is difficult
but here goes:
First check what underdog was mentioning about the hex hub. I've seen this happen on underdog's car and even after the diff rebuild, it was still jagged like you were mentioning, and I think we solved his problem by putting on the aluminum hex hubs from the 414M series.
If you're convince that it is the diff, then usually, as other have mentioned, it is the thrust bearing that gets griddy first. Because it is such a small bearing handling a lot of torque and load, and also being more outside, it tends to accumulate dirt and grease faster. Simply blast it out with some motor spray and put some grease back into it. I personally like to use AE black grease on the thrust (although Tamiya have you put the same diff grease on the thrust and diff balls). I put a tiny tab on the diff screw where the thrust bearing assembly will sit on, then put on the thrust bearing. I put a coat of black grease around the thrust bearing and work it into the balls inside the thrust washers. Then finally I put a small dab on top of the thrust bearing assembly (the side that will go into the diff outdrive first). This should ensure a very small thrust assembly.
For the diff balls and diff rings, I put a light layer on the diff rings as I put them on the diff outdrives, and then I fill each whole of the diff pulley where the diff balls sits with a bit of the diff grease (for the diff ring and balls I use AE diff grease, which is different from the black grease earlier). Then I set the ball in there and gently press them in with my fingers.
As you bring the two diff halves together, make sure the diff rings are seated properly on the diff outdrives (on the TA04, the diff rings are notched to match up on the outdrives). Sometimes they will move right when you are putting the diff assembly together. A small dab of diff grease on the outdrive before you put the diff ring on will help stick it in its place.
Once you get the diff together, keep turing the diff screw until the diff halves are against the diff pulley. The diff should not be tight yet. From here, every 1 turn or so, work the diff back and forth a little bit (rotate the diff halves) keep doing this until you lock the diff (don't over tighten, or you'll snap the plastic t-nut on the other side) and back out the diff screw 1/4-1/2 of a turn, and you should be okay. Lastly, take a Q-tip and clean off any excess diff grease or black grease (these grease attract dirt and dust very quickly) and...You're done! Sounds long, but after you do it a few times, you'll be able to rebuild diffs very quickly and smoothly
Since it was your first RC car (welcome to the hobby by the way
), I hope the information above will help you. Those of you on the board that would like to add and help out, feel free to do so....those of you that fell asleep because of my long post
I'm terribly sorry...hehehehe.
Speed Tech R/C