The synchros are a complex thing inside the trans. It's not just one specific part. There are 3 parts to the synchro. The slider, the blocker ring and the dog teeth on the gear.
The blocker rings are the parts normally changed in a rebuild, but if you grind the trans hard or continue to drive 5000 miles on a grinding trans, the dog teeth on the gears and sliders begin to deform. The next stage will be popping out of gear. These parts are expensive as OEM ford parts are approx $90 each. They are not included in a basic rebuild.