Don't panic... yet.
The hesitation at full throttle could be a number of things, some small... the Brat Hedz here will know for sure, but if you're on the lucky side of things, maybe even just a fuel filter.
I'm going to vote optimistically, on the griding sound, vibration, even loose 4th gear... linkage bushings or related, maybe?
Again - confirm with the Brat Pros first, there will be plenty of time to panic later.
Good Luck and Skill,
#89
1989 Subaru JUSTY RS 4WD
127, 669 miles
:-\ JUSTY in 'progress' pics: http://tinyurl.com/3y9h42