RedHeadedStepChild
Proudly wearing my MC1R mutation!
166k miles on a Toyota diff is young in my opinion.
U-Joints are a really good cheap/easy first place to start, however, I would only consider if the vibe you're talking about is a straight line on throttle/off throttle repeatable event. If what you are experiencing is somehow manifest in left/right steering I would rule out U-joints.
I can NOT get things to repeat with left-right, which has lead me to rune out wheel bearings.
The vibes are always at highway speeds. On or off throttle doesn't matter.
The grumble comes and goes, but is noticeable at lower speeds (25mph or less) typically off throttle is when I notice it, but it HAS been there while on throttle.