“Klunk” when shifting to R

Hi guys

My 79 Lebaron runs and shifts just fine when going forward, but when shifting into R the is a rather loud "Klunk" or sometimes a metallic "Klang" like the shift happens with too much force or something. Reversing works fine once the shift has been made.
From what I can read in this transmission forum I have a 904 or a 998. (I know it has lock-up)
The car has done 51K miles. Could it be something is worn in the rear diff or does the Reverse function need adjusting ?
Author: Toro67