All of the posts seem valid, but here is soemthing my mechanic said to me about some noises especially that 'low howl". Overtightening can cause that howl by mechanics in a hurry and not following torque settings. I had that issue with our brick- he "re-did" it-proper tightening-howl gone-in my case anyway. Not saying that's your problem.

My 88 wagon had the same after 200k, may be the pinion gear in the differential. Does the pitch of the sound change with acceleration? I ended up swapping a donor rear end & replacing all bushings at that time.

Any blown bushing in the drive train can transfer drivetrain noise to the cab. The bushing have two functions, positioning and vibration isolation.
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Disclaimer: be careful, work safely!!! If you have any doubts as to the safety of the work you are doing, consult a proffessional before you hurt yourself or others!!! I'm not a licensed mechanic, I have just been fixing stuff my whole life, patrick of

I'm going out on a limb here, but when I had rear end noise it was the last U-joint on the drive shaft. The noise was very apparent while on the highway it appeared and disappeared at various speeds. You can check for play in the U-joints just by crawling under the car and yanking / twisting the drive shaft, but to be sure you might want to spend the 30 minutes to disconnect the driveshaft fromn the pinion flange and check for smooth rotation of the U-joints.

MRE

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'82 DL - 150k, '93 945 - 96k