I'm not saying this IS your problem, but if your stock driveshaft has been modified, shortened, lengthen, a spacer added, check it for runout with a dial indicator in the van. Also check that there is sufficient sliding travel, where the driveshaft isn't 'bottomed out' at either end of the suspension cycle. Both can cause binding, and bushing wear, and seals to start leaking.
I learned a painful lesson years ago, after doing a rear axle conversion. I needed the drive shaft shortened, a different rear yoke, and balanced.
It was my truck, but had had me chasing a vibration (only at certain speeds) for months. Balanced then re-balanced the new tires, re-balanaced the driveshaft, new shocks...
It turns out the driveline shop really 'fornicated the canine' the lathe work must have been done by the shop's toothless floor-sweeper, the shaft tube was misaligned to the yoke by a whopping and easy to measure .09" or so. I took it back, a-gain, he could get it to balance out at the balancing machine's fixed rpm, but spin it higher or lower rpm in the truck and the shake was unavoidable.
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1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift
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