My 1998 astrovan is very loud driving over bumps, potholes, and road imperfections. 4 "brand new" used tires but still haven't gotten an alignment. Air pressure is all the same.

its a van

check its suspension

Did you fit the correct shocks? Did you fit them the correct way up? Did you prime them before fitting?

Your fault points to a definite shocks problem first and it's extremely unlikely to be weak springs. The next most likely candidate are one or more suspension bushings.

You might have broken the sway bar link, or the strut, or strut mount.

SImple solution would be the wheels need to be balanced.

Not so simple solutions are
(1) tires that have delaminated the tread or some other failure (Very bad, could result in a blowout, need to be replaced)
(2) Need front end alignment
(3) Rim bent from hitting pothole or curb.

If tires look ok, are not worn out, and don't appear to need replacing, I'd start by getting them balanced. Most places you buy tires today offer road hazard & lifetime balancing, so this may not cost you anything if you return to the place you purchased tires from.

Once on the balance machine, they should be able to determine if items 1 or 3 apply. That just leaves item 2 to consider.

caster,camber and, wheel bearings
have your car 4 wheel aligned this will adjust your rear caster and, camber.
have your wheel bearings checked

The bumps do initiate the shaking. When my 2006 Ram 2500 4x4 started shaking, I had just had it aligned that morning. A month or so before that alignment, my passenger side outer tie-rod broke while driving down a rock road. I replaced it with a heavy duty MOOG tie-rod end. Anyway, the same day I had it aligned I was driving back home from a job and towing my bobcat behind me. I hit a bump and the truck did the "death wobble" with the front end shaking violently up and down in an alternating fashion (one tire in the air while one tire on the ground and then vice-versa). I changed the tires first (which were worn-out 305/75/17 mud tires). Since the day I bought the truck new in Jan. 2006, I've had larger tires on the truck than the tiny 235/75/17s that came with it and I've done a lot of heavy towing with it and never had a problem until this alignment was done at Sears. I;m still trying to figure out if it was the alignment specs that they used or if the problem coincidentally started that same day. I also changed the steering stabilizer. Neither the new tires nor the stabilizer/damper helped the problem. So I changed the other outer tie-rod end and the upper and lower ball joints (all MOOG with grease fittings). I turns out they were worn out. So, the truck no longer shakes violently up and down when I hit a bump. That problem seems to be in check. However, now when I hit a bump the truck's front end wobbles side-to-side. I can do a few things to recover from it. I can hit the brakes pretty hard and sudden (which can cause an accident on the interstate), or I can **** the steering wheel side-to-side a few times (from 10 o'clock to 2 o'clock) and that stops it immediately...until the next bump in the road. Also, if I let my foot off the accelerator just before hitting a bump and let the truck coast over it instead of power through it, sometimes the truck will ride over the bump without incident. I found out yesterday on a 5 1/2 hour road trip that if I did 80 to 90 miles per hour and powered through every road blemish, the truck rode fine...no shaking at all. I suppose the high speed forces the tires to stay straight when they hit a bump. Lastly, to confuse my problem solving attempts further, sometimes the truck rides like a cadillac (for an hour or so) at regular speeds and no matter what bumps I hit. Then without worning, the front end will loosen up and get that sloppy feeling again and the wobbling comes back at every bump...until the next time it feels like tightening up and driving right again. I have no idea what to do now. I'm going back to Sears today to have them check their alignment. Other than that, all I can think of is to change the rest of the steering components and check the steering box for malfunction. I hope someone can help all of us Dodge owners out off of the road tested info that I've just provided. Dodge doesn't seem to care to address the problems with their trucks front ends, and we need a solution before people get hurt or die. I was almost run over by an 18-wheeler trying to figure out how to handle the shake. I can't afford to buy another truck. So I need to fix this one so I can keep working. If anybody has a concrete solution to these problems, please let us know. I've read 20 different opinions from mechanics on these blogs, but nobody has said yet that they fixed someone's truck with these problems and have since heard from that customer and everything is still working fine. Opinions are helpful sometimes, but they're also confusing when they're conflicting.

have you checked the fan clutch assembly?

I have the same problem but others said that replacing the $25 sway bar bushings fixed the problem. This solution helped several owners so it's worth changing.

rotate the tires and if keeps doing take the car to a diferent shop and make sure to ask for the lates or at least a laser aligment machine, before chosing next place, but firts rotate tires and drive car.

check ur suspension