Gear change noise

I've got a slight problem when changing gears on a '96 manual 3.9 v8. If driving gently there is no problem, but after accelerating hard in a gear or trying to change down quickly at a junction I get a grinding/vibrating noise.

first thought is the synchromesh, because sometimes it won't go in gear unless I lift the clutch in neutral first. However, sometimes the noise starts after the gear is selected and I have the clutch pressed or riding on it, leading me to think it is the clutch release bearing.

Any ideas?

Check your UJ's on the props.

Any play - swap 'em out.

What is the best way to test them, just crawl underneath and shake them by hand?