Yes, the D6 has the linkage on the outside of the transmission and the D2 has it on the inside of the clutch housing.

Not sure where the D4 has it. Need someone with a D4 or a D4 book! But it does sound like that interlock between clutch and transmission is where your problem most likely is. Of course the detent/lock plungers on your shift rails could be rusted and stuck. I think the lock plungers always have some springs on them, so if the rust was bad enough the springs might be gone and not returning the plungers or applying them as the case may be.

the linkage is inside the clutch housing and under the top cover. you can see some of it through the oil fill hole in front of the shifter. take the inspection cover off of the clutch and work the clutch handle and you should be able to figure it out. there is a pin that can fall out also. you might be able to lube the lock out bars a little through the fill hole. if all else fails you will need to remove the top cover.

Ok, I can see the bars and there's a cam or shaft in front of those. They appear to be on a plate. I can see the bar roll when I pull forward to back. What should else should I see?

try soaking everything in that fill hole with oil and just work things back and forth. try GENTLY tapping on thos bars to help get the oil into things. you need to get a parts book so you can see the exploded view. you checked the linkage??? right???

This is probably what yours looks like, had to take the shift forks out, gears and shafts and clean every thing and replace bearings. This was all caused by a leaking gearshift boot.

Mine leaked as well. Can I over fill to soak? Other wise what am getting into to remove?

i don't think overfilling will hurt anything. if you decide to try that i would drain the gear oil out and use cheap ATF because of it's ability to penetrate. but if you think it is rusty inside it would be best to take the cover off and inspect the gears and bearings. is yours a seat tank or fender tank model?

Fender tank mount..

Here's a link of a partial walk around after I got it running.

drain the old oil and fill it to the brim with a thin light oil or use engine oil cut with diesel and let it soak. occasionally rattle the gearshift back and forth in all gears.