1992 F150 trans issue?

When going 30Mph or over truck rumbles.

Hey, I have a 1992 Ford F150 with a 5.0L. When I drive 30MPH or over and take my foot off of the gas it rumbles. I know this is not a motor issue as that had a 100% rebuild done to it. This is a big issue for me as I want to go get the truck restored as it was handed down to me from my grandfather. Thanks.

often a bad universal joint does that, or worn motor or transmission mounts.