Hello and welcome to JustAnswer!
Disconnecting the battery will clear the fault codes that are stored and are necessary to diagnose the problem. Don't disconnect the battery again until the problem is fixed.
The first thing you need to do is see what codes are stored in the trans controller. If you don't have a code reader or scan tool or your own almost all parts stores will read codes for free. If you can have the codes read and come back and post them here I'll be able to tell you what to do to fix your van.
I have the codes, I just want to turn off the service light. The codes are:
P1784 sets when the battery has been disconnected, P0700 just sets in the engine controller when there is a code stored in the trans controller. The P0733 and 734 are stored in the trans controller.
When you first start the van and take off, as it shifts into second gear what does it do? Does it go into second fine?
Does it bang into second and stay there?If it shifts into second fine, what happens when it shifts into third? Does it rev up like neutral?
So it jerks when you hit second gear, and never attempts to shift into third?
How fast are you going when it jerks? If it's jerking into second gear it would be jerking at the first shift that it makes.
Or does it shift into second fine, and then rev up (like neutral) when it tries to shift into third?
If it shifts ok into second gear and then revs up when it tries to shift into third that's a sign of planetary geartrain failure of some sort. This would go along with the two codes you had set.
The P0733 is gear ratio error in third, and P0734 is gear ratio error in fourth. The overdrive clutch is needed for third and fourth gear, and if it doesn't come on for third gear it will slip and then fall back into second and stay there until the key is cycled.
This could be the clutch itself, but is more likely to be the overdrive clutch hub or a planetary geartrain failure. What normally happens is the weld breaks on the overdrive clutch hub where the shaft mates to it, then the shaft isn't able to transmit engine power to the geartrain in third and fourth gear.
If this was the case you would either need to have the trans disassembled for inspection and then repaired, or replace it. A used replacement trans would probably be the cheapest way to go, you can normally pick these up used for around $300.