Yesterday was a nice sunny day, so I took the '82 out for a little ride thru the countryside. Everything was fine as I ran thru the gears several times, the bike ran great, no problems or strange noises at all. Then, after I slowed and turned onto a side street and began to accelerate, as I went to catch 4th there was nothing, no gear change, grinding noises, the pedal moved as usual, it just wouldn't shift. My very 1st thought was that the clutch cable had broken, but I quickly realized it was working fine, with the same tension and feel as always, and I could pull it in to disengage and roll in neutral. I slowed and tried to downshift to 2nd, but again, no gear engagement at all. There was no lockup at the shifter pedal, just no catching of the gears. I took a good quick look and didn't see anything obvious, so since I was riding in a fairly flat area, and luckily stuck in 3rd gear, I managed to feather the clutch and get the 10 miles home.
Today, I was able to check things out a little more thoroughly. Again, the clutch cable seems fine, and engages thru the linkages to the back of the trans. The shifter is attached and tight at its' pivot point, with nothing obvious seeming to be loose or rattly there. It still has tension at the pedal, but there's no gear engagement. I can pull in the clutch and start the bike, and it rolls fine with the lever in, but letting out the clutch causes the usual lurch and killing the engine. So, it's 3rd gear or nothing.

Does anyone have a suggestion as to what's up, and where to look next? I'm really hoping I won't have to go into the transmission: after the summer & fall I've had I really need to catch a break!
Thanks for your help!