Chrysler Repair: Voyager van shakes badly at 40 mph, 4 digit numbers, surface changes

Question
I've had a growing problem; my 2001 mini-van shakes badly when I hit 40 mph. Based on your previous advice, I eliminated certain possibilities: I redid the front brakes, and it no longer shakes when I'm braking. I replaced all 4 tie rods, and balanced the wheels. I live in CostaRica, and cannot find a Chrysler-savvy mechanic here. The suspension feels a bit loose. I checked the motor mounts, they seem to be OK. I'm hoping it doesn't require replacing my driving box - that's really costly here. Any ideas what to try? This shaking is scary. Gets worse the faster I go.  

Answer
Hi Joquin,
It would be good to separate the tire/rim/bearings/suspension/half-shafts possibilities from the engine/transmission possibilities. The latter would be detected by the engine/transmission controllers but the former would not. How about a fault code readout for possible issues with the engine and transmission being the cause: use the ignition key:"on-off-on-off-on and leave on", doing that in 5 seconds or less elapsed time. Then watch the odometer window to see if the mileage reading changes to instead show any 4-digit numbers preceded by a P. If so let me know what those are and we'll go from there. It you only get "done" that means there are not fault codes stored in the memories of the controllers and we have to consider the suspension/half shsfts/etc.
How about noises when the shaking occurs? Anything noteworthy? If a noise, does it change if you turn one way or the other instead of going straight forward?
Have you tried raising the front wheels and letting the tires recover from the flats of sitting on the ground, then after a while (hour or so) try holding a metal tipped rod about 2mm from the center tread and fixed at the other end against the ground and then rotate the tire to observe if the distance between the tip and the tire surface changes by more than 1 mm closer or further? If so, that would be an out of round tire that could cause such a vibration. It may be possible to reduce that oscillation by 'match-mounting' the tires on the rims (taking advantage of any out of roundness of the rim by mounting the tire so its out of roundness is 180 degrees from that of rim. The tire could be balanced but still be out of round. If the vibration goes away as you go faster than 40 mph that too would be a sign of out of roundness.
Anti-sway bar bushings seem to be subject to wear out and certainly cause noise, and possibly vibration in the extreme. Those can be replaced easily if worn.
Is there any wheel alignment/front end shop that might look at the suspension for wear on the other parts of the suspension? Maybe the owner would go for a ride and see what he thinks it might be.
Are the cv-joint boots all showing no signs of a tear or grease leakage?
Please let me know if you find something of interest.
Roland