I am the son of Joseph C Jackson, who per newspaper clipping was a Captain in the TNG 181st Field Artillery in 1940, when they converted from being the 109th Cavalry regiment. Family verbal history has him in the TNG Cavalry, and then the Artillery in the Pacific. Another clipping has him in the 191st Field Artillery in 1942. We are Not sure if there is a typo, or if both are correct? Is there a history page for either unit, or can you confirm his membership please?
The book Armor-Cavalry Regiments: Army National Guard Lineage has a pretty thorough discussion of the lineage of the 181st FA Bn. The 191st Field Artillery Bn served in the ETO, so I would tend to believe it is a typo. There is a roster for the 191st and his name does not appear on it, which supports the belief that it is a typo. Newspaper clippings can be helpful when trying to trace WWII service, but they can also be misleading as in this case. Just in case you are interested, here are a couple more links regarding the 181st that may be helpful to you or others: 181st Field Artillery Battalion on AllExperts.com WWII vet in 181st Field Artillery Bn
If there is a roster for the 191st, then perhaps there is a roster somewhere for the 181st, or its parts, as listed in the Armor-Cavalry book you suggest? The Armor-Cavalry book lists two units as taken from the 181st FA, the 947th FA went to the Pacific, and the 34th FA went to Europe; and the Unit Citations Credit Register further shows specific islands, specifically Leyte, Luson, and New Guinea. Perhaps there is a roster for the 947th please?
There may be rosters for the 181st and 947th available online, but it all depends if someone has taken the time to do it, as was the case in the link I provided for the 191st. I have not been able to find much on the 181st FA Bn with respect to unit histories or rosters. I did find a couple of listings of a yearbook for the 947th FA Bn. However, one was from 2007 and I don't know how legit the other is. The price tag on the latter was $85. You could check with your local library to see if they can find anything that they might be able to get on loan for you. If you haven't done so yet, I would suggest you try requesting his Official Military Personnel File (OMPF) from the NARA.