99 voyager, 3.3L engine.

So how do you get the belt off when the tensioner is frozen?
(Working from above, I put a long handled 15 mm wrench on the bolt head and pulled towards the front of the car. I was trying to see if I could do the job from above on this one too, but there is no movement in this pulley like there was on the 90, so it must be stuck.)

And then what do you do to replace the tensioner? (Are we out of driveway mechanic territory yet?)
fanbelt

You might have to get on it with a breaker bar to get it to move. Eventually they end up moving. Make sure you are pulling on the tensioner pulley which is up and to the left of the crank pulley as you are standing on the drivers side looking at the sengine. They are way easier to do with the passenger front tire off. The tensioner is held on with one nut that is accessible from under the vehicle. A swivel and a bunch of extensions is all you will need.