Is my minivan really junk because of one sheared bolt?

A spinning stub is actualy telling you something useful. On these vans the lower shock bolt threads into a captive nut that's spot welded inside the bracket, and when it spins without ever coming out, it usally means the nut broke loose from its spot welds. That's why vise grips did nothing. You're not fighting rusted threads anymore, you're just turning a loose nut inside a pocket. Two ways to fix it. The cheap DIY route is an air hammer with a pointed chisel bit. Get under there, put the point against what's left of the nut, and let it hammer for a bit. The vibration breaks whatever's left of the welds and the nut drops out the bottom of the bracket. Then you feed a new nut in through the access slot next to the hole, a zip tie around it helps you steer it into place, and thread a new bolt in to hold it while you snug everything up. No welding, no drilling, maybe an hour of fussing.

If you don't have an air compressor, a shop can do the weld-a-nut trick for you. Any exhaust shop or mobile welder handles this kind of thing constantly, and it's a fifteen minute job for them. Around here you'd be looking at somewhere between fifty and a hundred bucks, way less than a diagnostic fee at a dealer. Call around before you tow it anywhere, some places will do it while you wait in the parking lot. The bracket itself can be replaced, it's bolted to the crossmember, but there's no reason to go down that road when the nut trick works. And your coworker's advice is nonsense. A 140k Grand Caravan that runs well is worth way more than one bolt extraction. These rear shock bolts snap all the time on this generation, so this isn't some rare disaster, it's a Tuesday for anybody who's done brakes and shocks on a minivan.

Did this same job on my old Voyager years back, and the one thing that made it easy was taking the weight off before you start fishing that new nut around. Jack under the axle or have somebody sit in the back so the shock isn't trying to twist out of your hands. Loose nut in a dark pocket plus a spring pushing back is how you drop a bolt into the gravel and spend twenty minutes on your knees hunting for it.

And before the air hammer, try a hacksaw blade slipped in beside the stub if you can see any of the old nut. Two cuts across it, hit it with a cold chisel and a big hammer like we did before anybody owned impact guns, and it'll split right off. Crude, but it worked on plenty of frozen stuff back in the day.

Can vouch for the air hammer route, did the same fix on my Sienna when a rear shock nut let go. Took maybe two minutes of hammering before the old nut dropped out of the bracket. For steering the new one into place I stuck it to a socket extension with duct tape, worked about as well as anything. Also hit the stub bolt with a wire brush after and the threads were still fine, so don't assume you need a new bolt unless it's chewed up.

Update: turns out the first reply was right, the captive nut inside the bracket is what's spinning. I don't have an air hammer though, so before I go rent one, has anyone done this with a hand-held hammer and a chisel on the nut edge? Also, when the old one drops out, are you just replacing it with a regular nut and bolt through the bracket?