I would suggest going about it this way:

Ask the owner of the shop to accept a post dated check. I'd tell him right up front that you think he's guessing and would like to protect yourself against his incompetance.

At our shop, if someone gets their car fixed and it comes back the next day, it is considered not fixed.

I would highly recommend taking your truck to the dealer (I know it's expensive, but who knows your truck better?) and get it fixed right, the first time.

1990 Montero RS (In pieces... for now)

KG6VNX