Hello and welcome to JustAnswer!
Did this start after doing any work to the trans? Changing the fluid, etc?
Have you checked the fluid level with the van hot, idling and in park?
If you start the engine after the van has sat, drop it in gear quickly, will it take off immediately? Or is there a delay before it moves?
Are you able to check these couple things at the van now? Would it be possible to start it up and drop it right in gear and see if it takes right off, and then check the fluid real quick?
Not a problem! Knowing these two things will narrow it down for us.
how long should it run before i check the fluid
For best results you would want it hot, after being driven enough to be at operating temperature. For our purposes here just let it run a couple minutes, make sure the level is between the full and add marks on the stick.
The first thing you'll want to do is have your transmission controller software updated to the current level. There was a change to the software to not let this happen. What has happened is the underdrive clutch volume index has risen too high. A clutch volume index (CVI) is a measure of how much fluid the trans controller is using to apply a clutch. The only time the underdrive clutch comes on for an up or downshift is at the 4-3 downshift, which is why that's the only time you feel it.
The root cause of this is the trans losing prime as it sits, to some degree. It could be low fluid level, a leaking low/reverse accumulator piston cover seal, a leaking low reverse piston, or a bad pump. There were problems with some pumps of this era which could be your root cause.
If you aren't feeling any delay when cold then have the software updated and the trans controlled "quicklearned". This will set the CVI's back to a baseline to let them relearn correctly. After these two things are done then the problem will be gone. If it does return or you start to feel a delay after sitting then you may have a pump problem or one of these internal leaks.
the mech. we took it to thaught it was something elect. in the trans. his is very good and never done us wrong but said it would be 1800.00 dollars. i live in a small town so he is like the city mech. and yes he is ase. does that sound like something it could be for that price