Hard to tell over a video but i'll throw out a few guesses from my experience.
Tires would be number 1. Mechanic may have rotated them and if they had some light feathering they can cause howling at certain speeds etc. Try rotating them and seeing if the sound changes etc.
Second would be something in the drivetrain. Either trans, diffs or t-case. Could be low fluid or since your mechanic just changed the t-case fluid. The fresh, thin, fluid could be causing the worn gears to howl.
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