You're referencing the front windshield wipers and not the rear wiper, correct? I know the rear wiper on these have problems like you describe, and I've read it could be a bad ground or the motor is simply dying.
I recommend checking for a bad ground - perhaps that could cause the problem you're describing. Or, it could be the motor itself.
It sounds like you are handy, so you should be able to troubleshoot the linkage a little more to see if it is binding. I've torn apart the motor/linkage assembly on the previous generation and know the linkage easily separates from the motor. If the '08 is like that, then I'd suggest disconnecting it manually moving it yourself. If it isn't binding in a weird way, I'd guess the problem is the motor is just too weak - either a bad ground or bad motor. Unless the van has been in an accident or had some serious work done, I would guess the linkage should be fine as I don't know what would naturally cause it to bind up, though it's worth checking first.
You may get lucky and find the part you need used on eBay or
www.car-part.com.
Please keep us posted as to what you learn!
Good luck
Dave