Malcolm wrote the letter in 1959, turning to his spiritual leader for some marriage counseling. In the correspondence, Malcolm details issues he and his wife, Betty Shabazz, had been trying to sort out for months. He asks Elijah Muhammad to step in, saying ... "I have complete faith in whatever way you wish or may chose (or may not) to take it up with her."

We're told the letter was originally obtained by Elijah Muhammad's son, who gave it to his attorney ... who eventually sold it to a collector roughly 20 years ago. It's changed owners a few times since then, and is now being sold through Moments in Time for a cool $75,000.

For the record ... Malcolm and Betty remained married until his assassination in 1965.