Hey y'all I'm new to f body cars usually for for g body and x body bought my first camaro it's a 88 was originally a v6 . Has a 350 on it has anyone had a vibration is starts around 70 mph at 80 it gets worse. The car almost feels like something possible grinding I'm thinking in the rear or transmission . It's a 5 speed shifts good expect for downshift from 4th to 3rd thinking that's a synchro issue . Just wonder if there's any common problem with these ? Also the car feels like it's almost bucking while accelerating something it's runs great though . Thanks for any information or options

 

Check your driveline,might be a u joint going bad.

 

New driver shaft and I joints had driveshaft checked for being balanced and straight all is good

 

Well those 5 speeds weren't designed with a 350 engine in mind for starters,maybe a bad mainshaft bearing or something.
Could also be something in the rear end,might be a good idea to pull the cover off the pumpkin and have a look see.