Gear noise during braking

My 1994 Jeep Cherokee Sport, 4wd, 4.0L 130k miles makes a sound like gears clicking together during braking. Sound has been getting progressively worse over the past few months. The sound - sounds like it is coming somewhere near where the transmission mounts to the engine, though I'm not sure.

Before I get reamed by a mechanic, I'd like to be greased up first. Anyone have any ideas what may be causing the noise?

thanks

check your axle joints.