vibration only when accelerating from 60-70

Dynamic wheel balancing should solve your problem...find a shop that still does this.

If you're getting clicking or crunching I'd worry about the axles~

probably should replace the side that's gone wonky, they are less than a hundred dollars~

A mechanic can check out the U joints on the propeller shaft. They grab it by hand and apply some force to see if it moves at all. Take it to a different shop for a second opinion.

It' s helpful Tenspeed to remember that if the propellar shaft doesn't move, it ain't the shaft or gear mechanism. No u-joints on this model. But it seems dynamic balancing would only be a solution to spend money on if the vibration occurred both during acceleration AND during non-acceleration. Roy, do you think if it's out of balance, torque might actually reduce the amount of vibration? Do broken motor mounts yield a vibration symptom range that is only between 60-70? or wouldn't the vibration from broken motor mounts be a more widespread speed range?

unless it's vibration cancelling mount, no way, it is an active component setting up the resonation or riciprocation...gotta be movin' man~

I was just about to post "this couldn't possibly be a japanese car~" but alas, it is! they are sticklers with balance (even part of the culture) so an intrusive factor has made it's way into the puzzle...'mud in the tires' is a hilbilly way of sayin' the the rims got gank on them~ perhaps you should pull off each wheel and inspect for the clingons-

this has to be a simple puzzle, and we are trying to make it more complicated that it can be. Obviously, if your axle boots aren't torn, they won't be the source of the problem...NEVER and I mean never have I seen a axle make a noise let alone a resonation....do you know how fast it would have to be to "sing"...the axle alone, I mean....it's preposterous for the axle to be thumpin' and pumpin'....this is the arena of what makes ground contact~

dynamic balancing will TRY to set up resonations, then the computer will ask you to make an adjustment half or one gram lead weights attached to the rim...spin it again...and by the time you've complied with all the fussies, you've got an extra three ounces on the outside...I know I used to be that tire buddy who did this for a living!

somehow can't imagine that they still use LEAD for weights anymore, probably half the cars have this...an environmental concern? should see what they make balancing weights from these days...?

"It' s helpful Tenspeed to remember that if the propellar shaft doesn't move, it ain't the shaft or gear mechanism. No u-joints on this model" --- I'm thinking the propeller shaft is the same thing as the rear drive shaft from the transfer case to the rear axle differential. --- If that's true, it's spinning all the time. If you are accelerating, the VTM-4 AWD system in the MDX attempts to predict when traction will be lost and apply power to all four wheels before slippage occurs by monitoring throttle inputs. That would send power to all four wheels from 60-70. That should send power to the rear wheels whenever the same throttle input is used at lower speeds unless there is something else that prevents it.

I bet it is not your motor mounts, because if it was that it would vibrate some what all the time. I had a very similar issue, replacing the axel did it. Usually, you wan to do both sides at the same time to not trick your transmission. I replaced the axel and no more vibration at 50mph, under Torque. Good Luck !