Transmission reverse causes limp mode until restart

The clue to a transmission specialist is the delayed engagement into reverse. The place to start is by reading and recording the diagnostic fault codes. This can't be done with a simple code reader because those only read codes in the Engine Computer. You'll need to find a mechanic with a scanner that can access the Transmission Computer to read the codes stored in it. That will also be able to show the "clutch volume index, (CVI) which is an indicator of how much life is left in each of the clutch packs.

When there's excessive wear in a clutch pack, it will typically cause slippage between shifts, then that is when it will default into limp mode. When it goes to limp mode when standing still or at very low speeds, a sensor or other electrical problem is the better suspect.

Given the age of the van, another good suspect is hardened rubber lip seals in a clutch pack. That would cause it to take too long to fill and engage. That should show up in the CVI numbers.

Also don't overlook low fluid level.