Hi Merc-
There are several things it could be - before you worry about the internals:
Remove the plastic covers so you can easily see all the parts.
Here are some thoughts:
1) Ensure that the bolt and spring that hold the shuttle in R, N or F is not loaded up with grit. Make sure that the area is lubricated. The shuttle should move easily. Ensure that the safety switch mounted on the shuttle isn't messed up and preventing free movement.
2) Moving down, make sure there are no obvious binding spots on the shuttle shaft from junk, dirt, rust.
3) Moving down, ensure that the universal is free to move. You may need to pull the shuttle shaft out to do this. If so just mark where it goes on the spline to make assembly faster.
4) ensure that the spline entering the transmission isn't rusted - this should turn freely in either direction
Let us know what you find after review.