1) Consider using stop-loss orders set at maybe 3 to 5% below current prices. If stocks continue to rally, as they probably will, congratulations! You get to keep the gains and roll up your stops to a higher price, ratcheting in gains the whole way up as you wait for a chance to "rebalance". When that 3-5% dip does come, you automatically move to bonds at that time. Conditional orders can automate all this, taking the emotion out of the moment. Also, it's not market timing; it's an algorithm that will probably/usually outperform an immediate rebalance. The risk you take in exchange for all this upside is 3-5%.
2) Consider selling calls against the shares you want to rebalance. Unlike #1, doing this by itself does not provide much protection against a correction - maybe a couple percent. However, it is a way to earn income from your rebalancing efforts. You can also sell calls in addition to having a stop by having your stop limit conditional order liquidate both your stock and the call position. As long as we're talking short term calls, like 30 days, your call will probably be cheaper to buy back in the event the stock drops 3-5%. If stocks stay rangebound or flat, you keep up the strategy and potentially earn years of bond yields just trying to sell your stocks. Win, win, or lose very small.