I made the mistake of not separating my pre and post tax 401(k) contributions while working. I am now retired and I want to know how and if I can take the approximately $75K (actual after tax contributions) out of my $1MM+ IRA and put into a Roth IRA now ? If so, what do I need to do?
From your description, it seems that you should have $75k of "basis" (i.e. after-tax amount) in your Traditional IRA. (As others have mentioned in comments, you might have had to report this in the year it was rolled over. I will assume that you will have figured this out.)
Having after-tax amounts in a Traditional IRA is tricky as you usually can't choose to take out only the after-tax portion. Withdrawals as well as conversions to Roth IRA must be "pro-rata", which means that the amount of pre-tax and post-tax money in the withdrawal or conversion follows the same proportion as in the IRA overall. So say 7.5% of your Traditional IRA is after-tax, and the other 92.5% is pre-tax, that means any withdrawal will consist of pre-tax and post-tax in those proportions. So e.g. if you withdraw or convert $1000, $75 will be post-tax and $925 will be pre-tax. So any withdrawal or conversion will consist overwhelmingly of pre-tax amounts, which will be taxed, and you may not want that at this time.
There is one way to separate the pre-tax and post-tax amounts in a Traditional IRA, but it involves having an active 401(k) (which I doubt you have at this point as you're retired). Some 401(k) plans allow people to rollover funds from Traditional IRA into it. If they allow this, then you can use it to "siphon" only pre-tax money from the Traditional IRA, as IRS does not allow rollover of post-tax money into a 401(k). This way you can rollover the entire pre-tax amount of the Traditional IRA into the Traditional 401(k), leaving behind only after-tax money in the Traditional IRA, which you can immediately convert to Roth IRA with no tax.