My 2003 V-6 Mustang grinds going from 4th to 5th at high RPMs but if I take it to neutral and hold it there for a second then gently put it into 5th it goes in smooth any ideas?

 

Synchros. Are you rev matching when at high RPMs? If not, you are putting a lot of strain on the synchros, and are wearing them out.

 

that's normal, don't shift hard into 5th you will destroy your syncro's. give it some time in neutral before you shift at high rpms

 

Yup your synchros are worn out. The transmission needs to be dropped & disassembled in order to fix it. It's a big PITA that synchros themselves are really cheap, but the labor to install it is kinda pricey.

But while the tranny is out you can take advantage of it by replacing the clutch too, if it's feeling a bit tired

 

synchros are toast