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I have a 96 Ford Aerostar van with the 4.0 V6 engine and 4-wheel drive.

It has started to make a strange noise/vibration that has my mechanic and I stumped as to the cause.  When we lift the gas pedal and decelerate, a fast-pitched vibration/noise will occur for about 3-4 seconds.  It almost feels like driving over those wake-up rumble strips along the sides of the freeway.  When the noise occurs, the van decelerates more than it would normally, like the van is dragging something behind it that makes it go slower.  At the end of the noise/vibration, there is a slight jerk (like whatever was being “dragged” is suddenly released) then it decelerates normally.  The speed range at which this happens is between 40-65 mph.  When I lockout the overdrive to force it to stay in 3rd gear, the problem is the same.

This strange noise/vibration started immediately after we had the following work done (by a tire shop):  new front tires, new outer tie rods, and a front end alignment.  I don’t know if this problem is related to this recent work or not.  When we took the van to our regular mechanic to investigate the noise, he discovered the front axles needed to be replaced along with the left inner tie rod, which he did.  This unfortunately did not fix the noise/vibration.

If anyone can suggest what this problem might be based on prior experience, or suggest a course of diagnosis, my mechanic and I would be very appreciative.  Thank you.  

I believe thats full time 4 wheel drive? Perhaps the tires are not the same height. Even if they are the same size some have more tread than others and if matched to a worn set can cause problems with full time 4 wheel drive. If so you could seriously damage parts like your rear differential. I would suggest changing the grease in the rear end anyway, that way your mechanic can inspect the gears for damage.