LEFT front brake intermittently grabs and/or pulsates when braking. ABS warning light may go on but not necessarily when brake is applied or pulsating. Dealer claims problem is left REAR speed sensor. My response is "Whaaaaat?"
Can a mechanical problem in one wheel's brake trigger an error code in another wheel's speed sensor? Illogical, but then, it's a Jag.

 

What code are you getting? That will tell you where the problem is. I would have to think the dealer read the code correctly. What makes you sure it is the front brake is the cause? The codes don't lie.