Drive it without the TT. When in an area where you can get up to speed such as getting on the expressway, you want to pay attention to how it shifts up through the gears. You dont want to enter the freeway slowly and you dont need it flat pedaled either. You want to be on it enough so that the shift points are noticeable. I know your problem is downshifting but start with this first.
It should shift from first to second relatively quickly then from second to third and finally when it's going fast enough the torque converter will lock up. The torque converter lock will not be as noticeable as the gear changes.
At about 55 or over the torque converter should be locked. If you accelerate a little it should unlock the torque converter and kick down to third gear. If you press harder it should kick down from third to second and be very noticeable. If nothing has happened floor it and see if it kicks down.
If it still doesn't kick down, slow down to 50 or so and move the gear selector to the 2 position. The engine should rev higher and the van will try to slow down. Stay on the throttle. Now move the shifter back to D and it should shift up again. Be sure no one is behind you when trying this.
After that start over doing it again but this time put the gear selector in 1 while stopped. Pay close attention to the engine speed as you start to accelerate. As it revs up and you are probably doing 15 or 20 mph move the lever to the 2 position. The up shift should be very noticeable. Stay on the throttle and when you hit about 45 or so move the shift lever to 3 or D (Drive) position and you will feel it shift again. You should have felt 2 distinct shifts (1 to 2 and 2 to 3). Continue accelerating until you feel a very another slight shift and that will be the torque converter locking up. It may be difficult to notice the torque converter lock up and it could be at 55 or 60 or so. I think a lot of those era vehicles locked at 48 or so under light throttle. If you have a tachometer you will also see these shift points, especially the converter lock up.
You are looking for it to kick down when you step on it. If it doesn't you have identified one problem. You are also trying to prove that it can be shifted manually. It could be an adjustment or it may need more serious attention which should be part of the trans rebuilders responsibility it would seem.
