Transmission Problem, My Baby's Stuck in "Neutral" Options

[ThinAir]

As I'm pulling out of a parking lot today, I hear a clunk from the rear end and the engine revs because there is no power going to the rear wheels. I've had a few issues with not getting it into first all the way so I figure it just popped out of gear, but no - it can't be that easy. So AAA brings me home and I check out the grub screws - nope, they are both in place too!

So that eliminates all the easy things and I think it points to something internal (and really bad/expensive) inside the transmission. It's a 73 with side-shifter tranny.

Here's what I know -
1. It seems to shift fine into all gears. I've visually confirmed the movement of the shaft on the transmission.
2. With one rear wheel in the air and one on the ground, if I put it in gear and let out the clutch the wheel in the air turns in the expected direction. There are some clunking sounds to be heard that make me think something isn't right with the synchro mechanism. It sounds like they are coming from the other wheel "slipping" as it is supposed to do.
3. There is no apparent problem with the clutch operation.

Before I pull the tranny, what am I looking for? If this set of symptoms sounds familiar, what do you think it is? Can I determine the cause by removing the side-shifter mechanism or do I need to take off the nose cone? Who in Arizona should I take it too? (I'm going to be in Phoenix this weekend.)

Since putting it back on the road on June 11th, this car has been a dream to drive. Fortunately I've got another transmission so there will be no problem with making it to the Red Rock Classic in September, but I'm still bummed about it.

[ThinAir]

Update -

Just like Royce said, something isn't turning that should. The outer passenger-side CV joint came loose. All the bolts are still there. I'll have to wait until I get the shaft pulled to see whether the pins are present and if the holes are wallowed out. At least it isn't something internal to the tranny!