1997 Ford F-150 I am getting a weird noise from the rear end

I had bought this truck from a rental company and it had about 230,000 miles on it and was in great shape. I have kept it in great shape and now use it for my appliance repair service so I also am putting on a lot of miles. But as of late I am noticing a weird noise coming from the rear end, and now it also sounds a bit like it could be coming from the trans. This will happen when I am going about 40mph, and the noise sound like a mild holing of sorts, almost like tires would make. And now the newer noise that it is making is when I am going down the highway and my foot is on the gas just ever so lightly to just about a cost, well the noise sounds like a heat sheild or something tinny is vibrating. I have been all over this truck to find anything that I can think of that would make the noise and I have found nothing so it must be something internal. I am woundering what parts could make a noise like these and if it is something I should get replaced. The truck runs great and is very dependable and I really don't want to get rid of it, or get jammed up one day out on the highway. I think maybe the toqure converter, or something like that? It will also give a bit or a wobble when the truck is slowing down under its own power. It must be something in the drivetrain!

Anyway if you can point me in any direction that would be GREAT!
Thanks For Taking Your Time To Read This!
Matt