Pre 88 vans didn't have the rear sensor, Walker makes jigsaw puzzle exhaust, so ordering the correct over axle portion (with or w/o o2) is how they got around one system fitting all years.

Bosal 2 made complete rear sections (->87MY,88MY->)
But probably not an issue as I believe the Bosal stuff is long gone, the Walker may also be in the same boat.

Rock Auto pricing can be confusing because in a lot of cases they are dealing with odd lots and cast-offs and inventory liquidations.... because of that, pricing isn't always the deciding factor (on quality) over there.

I have had really good luck with NTK and Denso sensors
On the other hand, I will never install another Bosch o2