Look (search) into 2nd gear synchros.. Mine did that (look for a post of mine, on the old boards, about my transmission)..
My particular problem ended up being the shift-linkage had snapped.. (but the synchros were bad, too..)
The slower I went, the easier to change gears.. and it was like no matter HOW much I was SURE it was in gear.. it would grind when I let out the clutch...
Ray