I got a problem with my 3rd gear. If i try to drive hard and change gears at a higher rpm then at 3rd gear it grinds. But when i'm driving slow and chnage into 3rd its ok. Could something like this be a huge problem like a syncro or something and if so would it be easier to repair tranny or replace oem or aftermarket.

 

prob a synchro ya.. and if u dont know about rebuilding trans then easier to find a spare used one and swap.