could be the syncros, or you could have the wrong fluid in the tranny. you should be running OEM, or Castrol Syntorq 75w-85 (exact equivalent of the OEM diaqueen)

 

Ok try this, when you shift from 3rd to 4th, pull it out of 3rd, let the shift hang a little before going all the way into 4th, then pull it all the way into 4th. In other words, let the sycros get a chance start spinning, then once they are up to speed shift all the way in.

If it still grinds, is either the linkage or something far worse, bent shift fork inside the tranny or something along those lines.

Report back and let us know!

 

Sounds like syncros to me. My 4th and 5th do the exact same thing!

 

Same on my car but Only from second to third. Changed everything and nothing fixxed it so I went to a shop and now I'm getting a sheppard trans the synchros were shot

 

synchros are a common problem. and since it just started happening i would bet that they are your problem. you can buy some time with the lucus trans additive, but uts just a band-aid fix. If you go ahead and pull the trans down you can save money in the long run by just replacing the synchro.