New starter has grinding noise after engine starts

I have a 1991, Chevy S10 Tahoe, 2.8 liter, 2wd, 5spd, A/C.

Shimming changed noise but did not eliminate it.

Recently the starter would not work consistently. It worked most times but sometimes it would not. When it failed to work, I noticed that when I tried to turn the engine over, all the dash lights and radio lights went out.
Note: this is the "original" starter & solenoid unit.

Troubleshooting:
Even though I have a new battery, I tested it first.
Both voltage tests but also battery and cranking tests, using a SOLAR BT7 test device, PASSED.
All was well.

The more I read in the Chilton manual and online, the more it seemed it could be the solenoid unit. Maybe the brushes were almost gone and therefore occasionally shorting out.

I found a new AC DELCO starter online for just $69.99.
ACD# 337-1003
GM# 88877102
Before ordering I cross-referenced the part number I needed, on 4 websites.
All websites said I needed ACD# 337-1003

When I removed the old unit, there was a difference in physical size between motor portion of the starter.
The paperwork inside the new unit's box had a tech bulletin TB-877, which stated the following about this starter:
" Despite the difference in appearance, these four starters (pictured above) have the same fit, function and increased performance over the earlier starters shown below"

There was also a circle with a slash through it stating, "No shimming required"

Day 1:

a. I disconnected battery terminals and then removed old starter.
I installed the new starter and used no shims, as per the instructions, which said "no shimming required".
My truck started okay and ran, but there was a slight grinding noise.
I immediately shut if off.

b. I disconnected battery cables again and removed starter. I installed the 2 original shims.
I then tried starting the vehicle and it started okay and ran. But the slight grinding noise was still there, BUT it was LESS NOISY than before.

Day 2:

a. Went to a local auto store (oreillys) and bought 2 packages of starter shims on way home from work.
b. Disconnected battery, removed starter and installed 2 of the thinnest shims.
c. Started vehicle and I still heard some slight grinding noise.

One co-worker presented the issue to his ASE mechanic brother. He said I probably have the wrong starter!

I don't believe so. To check with an offline source I stopped back at a local parts store (Oreillys) again. I asked them to look up my vehicle and they indeed show I need ACD# 337-1003.

NOTE: there is an email source for TECH SUPPORT inside the starter. I contacted them but I NEVER RECEIVED ANY RESPONSE!

After spending so much time on this issue, I am ready to try something else.

Since my old starter still has good gear teeth, but the solenoid motor's brushes are probably about shot,
I am thinking the quickest thing to do is just buy a new solenoid and install it on the old starter motor.

Any help is appreciated