If you move funds from stocks and bonds to cash, the transfer will not be taxable. The money is taxable only if you take (distribute/withdraw) it from your IRA, and the amount is not rolled over back into another retirement account.
You should check with your advisor, however, about any transaction or management fees for such a transaction, as these may apply and are likely to differ from one custodian to another.You may want also want to talk to your advisor about rebalancing your account in general; he or she can help you do this in a way that will minimize losses.
Consider too, that if you are taking substantially equal periodic payments (SEPP) under the amortization or annuitization method, you could switch to the required minimum distribution (RMD) method to withdraw a smaller amount each year. Of course, this would work only if you can get by on the smaller amount.
For related reading, see Bear-Proof Your Retirement Portfolio.
This question was answered by Denise Appleby.