If all of your holdings are pledged then you will not be able to initiate a rebalance. However you can initiate a partial rebalance if only a part of your holdings is pledged. The that have not been pledged from the holdings can be sold and the funds received from the same can be used for buying the recommended stocks to the extent of the funds received. To complete the whole rebalance you will need to unpledge the remaining stocks and buy the remaining recommended quantity of shares.