Gears grinding shifting 07 jk 2dr
 

Generally speaking there are two possibilities: The clutch isn't fully disengaging or the synchros in the trans are not working well enough under the conditions at the time.

Test the clutch disengagement by slowly letting the clutch pedal up in sixth gear at a dead stop while watching the tach. When the tach just begins to drop, the clutch pedal should be about 2-3 inches off the floor. I didn't look up the spec, it's approximate, the important point being that there is a significant distance that the clutch gets disengaged beyond where it appears to engage.

On the transmission side of this problem, the synchros can be affected lubricant age/type. Suggest changing the oil with the proper manual trans fluid, preferably a full synthetic (especially in cold winter weather).

You wrote about shifting slowly when it happens. In my experience with manual transmission shifting, there's too slow, too fast, and everything in between is just right. Unless the clutch isn't disengaging, suggest changing the oil and not shifting real slow.

Stay in touch, interested to hear back. :cheers: