I HAD THIS PROBLEM! I hated it... it used to only happen at 65+mph thought it was my tcase, got it rebuilt but it still happened. (i have an sm465 btw) and it pissed me off, after i got my 14bolt ff in, it started happening at 45mph. When i let off completely it would grind, when i let off and give it a tiny bit of gas it would do it when i let off and gas it. And it would do it with the clutch pushed in at 60mph+
I got a cv shaft and guess what, no vibes, no grinding. Happy sailing. I suggest finding a better driveline angle via shims or lowering your tcase a tad-- btw fill out your profile. If not that, then its probably something related to tcase and a bad chain in your 208
--about loading stuff in your truck, when i had my front axle in the bed of my truck, before i got my cv shaft, everything also seemed good, less vibes, less grinding noise, etc. So yeah... it could definately be your angles.