How much toe in are you running?
What power level?
What tires?
Which rear upper arm bushings?

Just curious, I don't have any answers for you except you can try to run closer to 0 toe if you aren't already and put something more solid than the stock sliding toe bushings in the rear upper a-arm of you haven't already.

 

 

Were both the left and right sides bad? If only one side was bad, on the failed side was the new dust boot that you used all snug and sealing all the way around?

 

Definitely don't do poly bushings in the toe links as it will bind up the rear suspension movement.

I asked about the rear upper a-arm because stock ones have "sliding bushings" that move fore and aft to do dynamic toe/caster changes. Them moving around will give you a bunch of toe braking and accelerating on track and put more stress on the toe link spherical *****.

For a track FD you should eliminate the front upper a-arm, front lower a-arm and rear upper a-arm sliding bushings so that your suspension holds its correct geometry.
The dynamic toe "sliding bushings" were put in to make the FD "safer" in an age before electronic dynamic stability control.
Once you eliminate the dynamic toe you get rid of that weird squirmy feeling FDs have when you go to wide/grippy tires.

 

 

FWIW my endlinks on my aftermarket toe arms seem to wear out about every 2-3 seasons and develop play. It just might be a wear point on cars that get tracked often.