Just picked up my '02 Ranger XLT a couple weeks ago, and it has a terrible quirk.
Every time I hit a puddle and splash a little water around, the belt slips off the front of the motor!
I'm thinking the dinks at the dealership installed too short a belt, but I wonder what you all have to say...

maybe you have a weak belt tensioner...

I thought about the tensioner, too. maybe I'll do both (all three if the Idler looks suspect) at once. Shame, though, being that the dealership jeee-ust put in a brand new belt.

this is one of the reasons most people fix for themselves...the dealership makes money on selling you parts...i for one got tired of having to figure out the problems myself and having to explain to the dealers what is/was wrong with the truck....i myself would do as you are saying...replace all the suspects...the item you don't change will become the weak link to the other new parts...