Need Help Asap
Are you sure the clumps of dirt are the cause of the noise?
It's hard for me to relate hitting clumps of dirt and bad pinion bearings. It's kind of tough to damage a pinion bearing by hitting something. Depending on how hard a hit it was you could have broken an iron side bearing shim if the car was sideways. But thats a hard enough hit that you'd most likely have a bent rim or at least be worried about that too.
I have no other ideas. Hope someone else can help you out.

