I don’t know if it would be a clutch problem if the transmission is only being difficult down shifting, I’m thinking worn or dirty syncro’s. If they are worn you’re into a rebuild but you may get lucky and hold off doing that by changing the fluid in an attempt to clean things up internally.
Sometimes dirt will collect in the threads that are machined into the syncro’s. These threads are there to cut through the oil so that the syncro can grab the next gear you’re trying to shift into, that’s so the two different gears are spinning at the same speed before they are forced to mesh.
Change the fluid two or three times in a row with short drives and lots of shifting in between, on the last fill, use synthetic 75W-90 gear lube.
There’s nothing to lose (other than a bit of oil) trying this and like I said you may get lucky and squeeze a little more time out of it before a rebuild.
Dan.
Here’s a picture of a syncro showing the threads.
Canadian "EH"