Your desired target is:52.0% US Stocks, 28% International Stocks, and 20% Bonds.
Right now, you are at:56.2% US Stocks, 27.1 International Stocks and 16.7% bonds,
You are not far off target at all. A rebalance is not really needed if you are comfortable with 5% bands. By that, I mean, being comfortable with 5% over or under your target . In this case, that would be a range of 47% to 57% for US stocks and you are in that range.
I just picked a common size band (5%) that keeps your risk level pretty close to target and which allows market ups and downs without constant rebalancing.
If you want to rebalance back to target, say just for the experience of doing it, I would only take a little from the 500 Index and add it to the Bond Fund in your 401k. That is the easiest approach. Your bonds are 3.3% below target. If your portfolio is $50k, then simply move 3.3% of it ($1,650) from 500 Index to Dodge and Cox in the 401k. It does not matter if you get exactly back to exactly 20%. Just somewhere in the neighborhood is good enough - the market will change it the next day anyway.
Another approach is to simply send a little more new money to the bond fund for awhile. If your current contributions to the 401k are 60% 500 Index and 40% Dodge and Cox, change it 50/50 for a few months and see if that brings your bond allocation back up. Or change it to 20/80 for a more dramatic change in a shorter time. Then change it back when the bond fund gets to a place you are comfortable with.
Not in your question, but here is a way to avoid a lot of rebalancing. First you set your portfolio to target (or near target). Then you send your contributions in at the same target ratios.
And example, say you are putting $17,500 in a 401k and $5,500 in each of 2 IRAs. That's a total of $28,500 a year.
Send 20% ($5,700 to bonds), 28% ($7,980) to international, and $14,820 to US stocks ($11,856 to 500 Index and $2964 to Extended Market). You can round these numbers to something convenient if you want.
401k $17,500 - $5700 to bonds, $11,800 to 500 Index
Her Roth IRA - all to International ah....here's the problem...you do need another fund in one account to make this work
His Roth IRA -
See if this works better for you:
Flip Flop the funds in the IRAs to:Her Roth IRA
27.1% total International
9.2% Total StockHis roth IRA
8.6% Extended Market
0% Total International
And do this for your contributions:
401k - $17,500 - $5700 to bonds, $11,800 to 500 Index
Her Roth IRA - all $5,500 to International
His roth IRA - $2960 to Extended Market, $2540 to International
If your international gets too high or too low, exchange it with the Total Stock Market in the same account.
Your concern about using fewer funds to get to Admiral Status is unnecessary. You can do that and be off target (but maybe only in the amount of extended market you have). Or you can add another fund and stay closer to target.
With a portfolio under $100k, it probably does not matter. Your contributions are growing the portfolio faster at this point that your funds are growing it. Right now, if you have your stock to bond ratio halfway right, it doesn't really matter that much what you are invested in because your portfolio will grow 20% or so just based on what you are saving.
If you really want to get to Admiral funds faster, drop the extended market for the time being. That should make it work.