So I replace all the plastic parts on my shifter plus the spring as well. After it was going amazing, but slowly pushing down to put the car in reverse was becoming a problem. After popping it out of reverse it wouldn't spring upward back normal so I would have to pull on the shift knob to get back into place
I took everything out and found this little piece inside my gear box. What's and help?

 

Upper shift housing alignment dowel.

Its why your shifter wont pop back up.

 

Is it hard to reinstall?

 

Not if you consider removing your transmission difficult... Transmission needs to be removed, then this can be re-installed.

 

Ouch!

I've regreased my shifter.. never knew it was possible to F that job up so badly.

 

It was likely not left out by the OP, but from a previous job.

Only need to lower back of trans to replace it. Check majestic diagram for where it goes.

 

They left a pin --- which should be inside the transmission --- on the gear shifter --- inside the car's interior? Wow!

Is it Number 4, on this diagram? Thanks!

 

It is not #4.

It goes between the trans housing and that B-34 top plate.

Need to refer to another diagram, it is not shown here.

 

Thanks! Maybe #15, in this diagram?

 

#15 is correct, there are two of them in there.

If either are missing, the top plate will not center itself over the shift pawl, #5 in the first pic. This will cause the shifter to stick in the down position when engaging reverse gear.