Please refer to IRS Publication 571 for 403b rules. To delay RMD, I believe that you should be working for the employer that corresponds to your 403b and not be retired from that employer. Your plan administrator should be able to assist you.
The latest Pub 571 that I find says, " You must receive all, or at least a certain minimum, of your interest accruing after 1986 in the 403(b) plan by April 1 of the calendar year following the later of the calendar year in which you become age 70½, or the calendar year in which you retire. [bold added] Check with your employer, plan administrator, or provider to find out whether this rule also applies to pre-1987 accruals. If not, a minimum amount of these accruals must begin to be distributed by the later of the end of the calendar year in which you reach age 75 or April 1 of the calendar year following retirement. For each year thereafter, the minimum distribution must be made by the last day of the year."
Note: You always need to take RMDs from your IRAs whether retired or not. Your IRA administrator should be able to tell you the RMD amount needed. Normally, your combined-IRA RMD is based on your combined-IRA balances at the end of the previous year.
