I have seen several 350s with the stock trans and tcase....it cures the problem many people have or perceive with the 230 AND adds power...quite a bit of it.
Driven above 50 mph for distances of 50 + miles, especially in warm weather, will make heat in many NP200 cases...there are some that dont have the heat problems...there are others that do...mine does.
The tcase is roughly below the rear part of the cab...so a noise under your seat could be coming directly from it OR the trans even. Not being able to hear the noise limits the ability to diagnose this...is it a grinding or something else? Is it from just above idle and up or from, say 30 mph or so and up?
The sticks moving apart....they do get loose at times...and they can move a little under the torque of the drivetrain...more from the former than the latter by far. The shift linkage, when tight and not under big torque input, doesnt move in that way...not in any that I have seen anyway...the linkage is pretty solid if tight is my point.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!