I have an M20 that has a 2083 shift body attached. What is the difference between that body and the stock 3138? Do I need to use the base plate? It is currently hooked up without a plate, but the shifting is not smooth. Not sure is the rods are part of the problem, they are not correct either.

 

When you say shift body do you mean the shifter rod or the shifter itself? As long as the rods don't interfear with one another and are adjusted properlythey shouldn't be the problem. If the trans is moderatly worn out it could be the syncro's.

 

The shifting was ok when first installed, but now I can't downshift to 3rd at all. It just grinds, like the trans is not in a neutral state.

 

It sounds like just a simple 3rd/4th shifter rod adjustment. Assuming you have the factory style shifter rods that use bolts to hold the position of the shifter rods in place, it might have loosened up.