I experienced a similar problem in an 84 500 SEC. A brief delay, a bit of shudder, followed by engagement and normal operation . My problem dissapeared after a fluid/filter change. I am often frustrated by replies to posts that offer such simplistic an possibly invalid answers, however, I did nothing other than the above and now the automobile engages immediately and without incident in reverse. I was recently speaking to a transmission specialist with forty years of experience and he mentioned to me that he is still quite often surprised by the ways in which a transmission can surprise you. Good luck.