You "checked and adjusted that big nut on front of diff"?

That nut is not "adjustable". If it was loose, you had a problem. If you tightened it, you have a problem. It sounds like you may have a pinion and or pinion bearing problem. Noises that change with vehicle or engine speed are usually not a mount or bushing issue.

Change the rear diff fluid. Is is sparkly? If so, that is your pinion bearing coming out one sliver at a time.

Good luck,
Michael