Try the simplest fix FIRST.
RE-torque your wheel lug nuts to 100 lbs.
I swapped over to a set of freshly BALANCED rims and tires on my 98 Safari AWD. It drove perfectly for about 500 kms but then vibrated exactly like you describe. I had to re-torque the chrome steel wheels (three times) before they settled down. The lug nuts loosened up enough to give a vibration ( just like a wheel out of balance ) and it was especially noticeable during braking.
Wait till the ABS on the Astro just goes away and leaves you rolling like yer on ice, You'll need the pee to rinse out the brown stain!!
It's why many of us disable it. It's dangerous when it gets like that. It's kind of an old post, hopefully the OP has it sorted out by now. ( It is nice when people post the fix to the problem....HINT HINT
I took my old 97 in to a shop for the same thing .. I thought either sticy caliper or warped rotor ........ turns out one of my idler arms was about ready to fall off . I'm glad they caught that one . On severe braking you would feel the whole van bouncing around but for some reason it didn't affect the steering wheel as much as you would think . I had them replace both idler arms the shaking went away
I have the same sort of issue. If I don't drive it for a few days after putting it away "wet", it seems to shimmy a bit for the first few stops then goes away for the duration of the drive. I chalk it up to dirt and dust. I bought Akebono pads which are softer than others and believe them to to contribute. Every other part of the steering has been checked and rechecked.
My van shook/pulsed under braking. I thought it was the rear brakes (disc) because it didn't pull L|R under breaking and sounded like the rear (empty cargo makes a lot of noise). Did the front discs/pads - went for a drive, bam, no shake. Replaced the rears when I got back because that was the plan anyways.
Pads weren't particularly bad, it sat for a while before I bought it- maybe corroded, maybe warped, really can't say...not a problem any more :chevy:
(holy crap I didn't mean to necro this thread I got played by "Random sidebar threads" :whack:, sorry)