I don't know what's wrong with your truck, but it sounds like the shop is full of BS.

But, they have your money and your truck's not fixed.

Be forwarned: A postdated check gives you NO leverage. Your best option is to talk to the owner of the shop. Remain calm and tell him you don't think the problem with your truck was diagnosed correctly. Ask him what he recommends. Chances are good he'll just sit there or come back with more some silly BS.

Keep your cool and tell him that you don't think his plan is acceptable (unless it is) and you'd like to suggest an alternate plan.

Get him talking, and you guys will probably come up with something.

Make him your enemy and it'll cost you more money to get it fixed elsewhere.