That's due to "hard" water. I use a Mr. Clean AutoDry. After you wash your truck, use the "spot-free" rinse. If you use a carwash, also use a spot free rinse. I bought my AutoDry at Sams or Costco, can't remember exactly. Here's what it looks like:
Amazon.com: Mr Clean AutoDry Car Wash System Starter Kit: Health & Personal Care