Although you can go back an add an extra shim after break in if you feel the need, it isn't a must do as long as they were shimmed well in the first place.

Are you certain both diffs are clicking ? Usually the rear one will have issues before the front. I'd open the rear one first.

I just helped a local last week who's rear diff got chewed up. His shimming was correct and there were no hairline cracks in his rear arms, rear chassis brace, or rear gearbox. Turned out that he had his slipper WAY (locked essentially) too tight, which will chew up the diff pretty quick.

Also, hard braking is really tough on the gearboxes as well. Most dial their braking strengths down on their radio settings. I run like 90% brake EPA with a negative brake curve on top of that as well. To test, on the fastest part of your track, if you hit the brake hard, your car should still take a few feet to come to a complete stop.