I had to replace the idler pully this past spring but have no records of when it was last replaced. My guess is much longer than 50k ago. i was amazed that when my pully let go the front berings were released and the whole outside assembly was leaning to the passenger side. but it held the belt for another 10 miles. changing it out was a snap. the clearence was tight but i took a 6mm allen head socket to remove the center bolt.
if you have a bouncing problem the tensioning spring or the pnumatic shock my have weakend. remove tension from the spring side, slide the belt clear and check to see if your pnumatic shock has gone wimpy on you. as i understand it the springs also have a lifespan too.

little tip. the stock lug nut tire iron fits quite nicely into the hole on the top side of the upper spring tensioner mount. use the tire iron to release tension while you remove the upper long bolt that comes out by injector #1. and again use the tire iron to pull the spring back up when you are done.

__________________
sold
1987 300 SDL 318K
126.125 (OM603.961) (722.321 trans) (head#14)
Born on date 11/07/1986
2.5" Flowmaster Exhaust
flex-a-lite dual electric cooling fan conversion

now have a 1985 toyota diesel pickup