My 06 4.8iS had just developed steering wheel vibration between 70-80mph. I have yet to check it out.

You think its balance or driveline related?

what happens if you put it in neutral while it's shaking?

Suggestion of alignment was because you had lowered the vehicle not as something that would be the cause of your problem.

As I understand, your X was fine after the new tires, wheels and lowering for some length of time. The steering wheel vibration has become progressively worse over some period of time.

The vibration happens only when cruising between 45-60. It never starts at a lower cruising speed or a cruising speed over 60.

Does the vibration continue if you accelerate from 60 to 80 or so? Does it matter if the pedal is on the metal or will it happen with slow acceleration?
How many miles after the changes did the vibration begin and how many miles has it taken to get to the current level?

If your balance and alignment are both good, check the air pressure, if the pressure on the 4 tires are different then you'll develop a vibration in the steering. If your air pressure is consistent, find an empty parking lot/straight away and accelerate then make a hard stop, if you dip excessively then you need shocks/struts. Also if while braking your wheels turn to either the left of right that corresponding side may need a tie rod. Another thing you should do is check and top off your P.S. possibly replace the clamp (those single use wear out and cause a pretty significant leak which can cause steering issues) Also use white lithium grease and spray the rack and pinion. Let me know your findings. 14 years working in a tire shop, you learn that even the oddest things that may not be directly related can cause issues.

Also just an FYI if you have 2 different sets of tires or if the tires have a significantly different amount of tread left that can cause steering issues because for example if you have a set of tires with varying tread the coefficient of friction on each wheel, because one set of tires is gripping larger while the other may be gripping less causing a very slight slip. Also if you have cupped tread wear or one side tread wear that will cause vibrations. Some other things you can check are your brakes/rotors, if pads/rotors are excessively worn, aside from being a safety issue it can cause shimmying in the steering. Frozen caliper can also cause this.

I think the odds are extremely low that anything that you have mentioned in this or the previous post would cause the problem or the severity of the problem OP is experiencing.

A vibration that happens only in a given set of conditions is a harmonics issue. At a certain small range of speed all conditions are just right and a vibration occurs. For that to happen something turning or loose must vibrate. In this case that would mean something in the front suspension, tires, wheels or driveline is the problem. Often the same conditions will reoccur at 2X the speed.

Assuming the entire front suspension, tire balance, bent wheel and a deformed tire have been eliminated, I would move on to other potential cause such as CV joints and Guido (either of those should show other symptoms).

That said, because of the conditions and the miles on this X5 I am still leaning toward a suspension part(s) that is wearing out. if that is the case the vibration will expand to a wider range of conditions and just as it has, get worse. Vibration, especially those that are severe will wear or damage associated parts and should not be left unaddressed.