Further update:
I finally had the solid diff mounts installed. No noticeable increase in NVH but I already had a solid diff brace installed, which at the time did make diff whine noticeable, which is something I've learned to live with and ignore.
Summary of installed solid mounts:
- Sold rear subframe bushes (RSB) - no increase in NVH.
- Solid diff mounts - no noticeable increase in NVH with an already installed sold diff brace. There would likely be some increased diff whine without a sold diff brace but would be bearable.
- Solid engine mounts - very minor (easily bearable) increase to NVH. Becomes unnoticeable.
- Solid trans mounts - introduces considerable NVH but just bearable.
YMMV depending on your tolerance level for NVH.