Gear shift selector "thumps" into reverse or park from drive

Hi, I'm James. I just bought a Diamante about 2 months ago and it has done me well so far, but one of my biggest concerns is that the gear shift selector needs to be realigned. Through a bit of research online, I found that the way my car shifts from drive or reverse into park (makes a loud "bang", almost like the car is getting hit in the rear), that the gear shift selector may need realigned. A mechanic friend of mine told me without looking at my car, that all the problem is, is that there is a nut that needs to be tightened to tighten the gear shifter and he said its located right above the transmission, he said to have someone push the gear selector into park as far forward as they can, and while they are doing that, to tighten the nut. I was really hoping you may be able to help tell me to find where the nut is located exactly, or how to re align my gear shift selector. Thank you.