Before you rush off to the dealer or another shop you might want to look ahead a little in that this might end up in court, if you want. You need to give the shop a reasonable chance to fix what may have been an honest mistake/best try. If you don't then you will not have much of a case in small claims court. (my advice only from experience, not legal advice)
I would do what 52degrees suggests and give them one more try, then attempt to work out who pays what. It appears that they replaced a perfectly good part which you might want back in the truck. If you need to pay a bill you disagree with in order to get your truck back, make a note on the bill that you dispute the charges. Spending time in court is not what the shop wants so that risk may promote a fair resolution, just don't go around threatening them though. Remember flat rate means about 30% more time charged than actually spent on the repair, so there is some room to negotiate. You also should check to see if your state has any laws that do not allow post dated checks. Also be prepared that the PD check will just go through the bank anyway, most don't really pay much attention.