I just went back 3 years on the search and so far I think this is slightly different. The symptoms, up to 80km/hr smooth as silk, at 81 -120 a vibration that gets progressivly worse, then at 130 i'm a) going to fast and b) everything styarts to smooth out, this was worse yesterday on picking up relitaves from the airport, the inlaws sat on the rear bench and had there lugage behind them (all the additional weight was behind them) and certainly notived the vibration more. the vibration is most noticable between power loads, that is if you accellerate slowley it's not to bad, push hard and it's less noticable, however just feather the throttle and you notice it much more. Also going down a hill in N starting at 100km/hr it rides to 120, then by clicking it into OD the vibration is worse so free wheeling it's hardly noticable once the drive line is engaged it's back, this led me to the shafts and UJ's, the rear wheel bearings feel tight, the UJ in the main prop have no noticable play, the fronts had new tie rod and upper ball joints and the bottoms were solid, they got another squirt of grease just in case (it went through a safty a month ago and the 4000 has made no difference to the vibration) I almost think it might be a tyre balance if it wasn't for the fact that free wheeling it goes away and the tires rotote at the same speed. the vibration does not go when going through a curve either left or right so it's not changing when you load up a bearing. ????? suspension, I have to say the van is very tight on the road, the old van used to be a bit of a handful on off ramps at 100 but this one holds very nicely. The CV's feel tight and have no other way of checking them, the front prop also has tight UJ's and also earned some additional grease. The main prop is an aluminium affiar and does seem a little sloppy on the out put from the transmission, it feels liek somethings out of ballance and it's speed related as in lower gears with the engine spinning hard it does not happen. The transnission x-member has no play and the bushes look good, I was hoping to see something obvious like a worn UJ or a floating bearing but so far nothing , the brakes produce no vibration whatsoever. So I'm starting to think about blindly changing things, starting with the prop shaft and maybe the front shafts, the old van uad UJ's up front so that was easy to rule out. Any help would welcome.

 

It still sounds like a U-juint problem, may be tight and still have a bad bearing inside.

 

AWD it sounds like?
is large ugly weight hanging down on arm still on rear end? dynamic vibration damper.

start with spin balance of tires. tires react differently under load and coasting. may be broken belt or weaken sidewall.

any bad, broken, leaking shocks?

agree with bertha66, then on to U-joints. run at 100 kph for 30 minutes or so. park and feel check the u joints for temp, anything much hotter than air temp and hotter than its partners, its a bad one. front and rear drivelines and stub driveline transmission to transfer case.
how long since u-joints replaced?

if ok, then I would drop the rear end plate, drain the rear end, check for any cuttings, wear pattern on ring-pinion?, new syn. 70w90 and check preload and front pinion shaft bearing for looseness-wear.

next is transmission tail shaft bearing, I can't find specs on A4LD 4R55 tran but would guess no more than 0.015" or 0.4mm side play. also check side play on transfer case in and output shaft bearings.
you mentioned some concern about tranny tail shaft looseness.
call Ford shop for tail shaft side play specs. most auto tranny shops will try to get you in for "The Works"

 

Now my journey is 140 km each way to work and today after the van being empty of inlaws and suitcases itfelt much better for the first 20km or so them it got gradually worse, I'm guessing I noticed it more 2 days ago as we didn't stop at the airport as they were already waiting, making the journey longer, I'm going to swap the prop for the spare one and see if there any difference good or bad, if not I'll try swapping tyres around, one thing to notice is that is certainly more noticeable at the end of the journey than at the start. Shocks look good and within 5 years old (that doesn't say much but there all matched monroes with no leakage and the bolts around them are far from rusty). I do have another pair of new shocks for the rear if this starts to be a thought but as I mentioned previously I'd like to avoid blindly changing things, i'll have a grope of the prop when I pull up in the morning and see if its heating up at all.

 

Try the motor mounts. Aerostar's lose them very fast.

 

sounds like tires if the problem changes as they warm up...i.e. distance puts more heat in the tires. I'd check the tires for an out of round condition first
Gary