I'm not Tony , but I'd say yes , you need to replace it . There are two
threaded and slotted pieces , one on either side, under bolted caps  . They
hold the bearings the differential unit rides in , and they are used to
adjust the clearance between the ring and pinion gears . That piece (if it's
the lock , but they both appear to be in place) is what keeps the adjuster
rings from rotating .
 You also need to completely disassemble that diff and inspect every
piece - including removing the pinion shaft . That 100 grit metallic dust
was ground off of something in there , it ain't normal . It looks from your
pics that the ring gear is OK , so we can probably assume the pinion is too
. You might be a very lucky guy , having caught this before it totally
trashed your axle .
<< I ain't nuthin' but a shade tree mechanic , but *nobody* works on my cars
unless it's an emergency >>

And, so, do I need a replacement for it or what?

i

when i look at the piece, it looks like one of the side adjuster locks broke
off.,, If so that would throw the ring gear adjustment out of whack.

Tony

I decided to take pictures as I go along. See

http://igor.chudov.com/projects/Automotive/Drivetrain-Noise-In-Dodge-RAM-1500-Pickup/

Some findings as I opened up the differential.

1. It is full of gear oil. (means my oil change 2 years ago was successful)

2. On the bottom there is some amount of of, like, 100 grit metal
dust, settled down.

3. There was one foreign object on the bottom, see

  http://tinyurl.com/zr8zg

4. The gears and such, do not seem to have any unusual damage.

i