something is wrong???

The clunk sounds like it could be the slip yoke on the rear driveshaft where it goes into the transmission. Lubing the inside of the slip yoke with good grease will usually cure it.
A common problem that causes the OD light to flash is a bad speed sensor on the top of the rear axle housing. Does the speedo work ok? ABS light not on?
What size engine in your truck? Does the tick go away when it's warm?


