I know this is a reed problem, but I just don't know how/where to adjust the reed to fix it so if someone could tell me how that would be great!

So my high D (not the highest D, the one just above middle C) is REALLY sharp, especially when slurring down from higher notes. If I loosen my embouchure it comes down, and when it does, it sounds like a completely new note (it makes a really drastic jump between pitches). Is there anything I can do to my reed to fix this problem? Thanks!

It sounds as if you are using too much lip pressure as you go to the higher notes.

Maybe a slightly harder reed plus using more air support instead of lip tension will improve this. I suggest getting used to the breath & lip pressure for this note (concentrating on the former & using embouchure for the fine control) by playing the D in isolation. Then gradually play higher notes using mainly breath support. With practise the D should come right & the quality of the high notes will improve.

Good luck.

I don't think that's it, because I don't have this problem on every reed. Plus, none of the notes around it are that sharp.

Might be worth checking to see that the wing joint key/pad that works off the 3rd finger ring ring is seating properly. If it's slightly off the tone hole it will affect the D.