Hello again JA,
First of all, what you have withheld from your pension in taxes is just an estimate of what you think you may owe at the end of the year. You won't know your actual tax amount until you do your taxes.
But the biggest problem is that you said your total income was actually $74,747. So if your pension was only $54,450, then you had an additional $20,297 in income that I assume you had no taxes withheld from.
If your total income for the year was $74,747, if you filed as single with two dependents, you would owe tax of $11,619. If you only paid in $4,200, then you would owe an additional $7,419. These are just rough estimates, but it's close to what you say you are being asked to pay.
The problem is that first, you did not have enough withheld from your pension check because you were claiming married with 3 exemptions. The second problem is that you had additional income of $20,297, and you had no taxes withheld at all on that income.
You really need to start having taxes withheld from the additional income that you have, whether that be from SS or other sources. Otherwise you will end up owing again next year.
Thank you JA