Stuck in Gear, new trans stuck in 5th gear Options

[astronomerdave]

I just bought a newly rebuilt side shifter to upgrade my '72 tail shifter. Upon initial assembly of the shift linkage I had the trans in N, let the cabin shift lever find its place left-to-right (which was mostly upright), held it centered front-to-back, and tightened the bolt on the forked part under the shifter plate.

I shifted into R, backed out, then 1, and took it for a test drive. 1-2-3 no problem. But I couldn't find 4th gear; it looked like the shifter wasn't moving over far enough. So I figured it was just a simple shifter adjustment. Back in the garage I loosened the adjustment bolt, moved the shifter over a bit, and re-tightened.

Sitting stationary in the garage I checked R-1-2-3-4-5 and then it was stuck. I couldn't move it at all at that point. (I actually thought it was stuck in 4th but now that I look at it, it could be 5th.)

I removed the linkage and tried grabbing the little input shaft arm right on the trans but I cannot move it. There is a little bit of front-to-back play but not enough to change gears and no up-down movement.

The questions:

1) Should this be possible and how is this possible to get stuck in gear?

2) Can I remedy this at home?

The place I bought it offered to pay for shipping both ways so they'll take care of me if I have to do that. They suggested I first try to remove the two nuts securing the cover around the input shaft on the side and move some kind of fork in/out of some kind of detent. Not being a transmission guy I don't quite understand what I'm looking for. Plus, as I suspect, if the shift arm can't move whatever it is, how can I do this with my finger? I did remove this and pulled out the input shaft with its attached three-tanged arm, but not sure how to proceed now.

Thanks...