First breakdown
 

I had just got off the freeway when the engine started squealing.

The spring on the tensioner pulley sheared off about 3 coils down from the top.
I was able to limp it over to a buddies house, and get a ride home to pickup my spare.

I had a spare because the one that broke was brand new, recently bought from pelicanparts. The pulley was making noise so I figured why not replace everything. It only had a couple hundred miles on it.

Another aftermarket part to beware of?

definitely. I recall a post by someone who had an issue with an aftermarket spring. I did a quick search and didn't find the thread but I did find where 83W126 said (tensioner) springs are known to break:
http://www.peachparts.com/shopforum/3962299-post6.html
In his words when asked of one should replace the spring he said: "I would, its definitely not unusual for them to break, and you are at minimum stuck for several days if you don't have a spare, and at worst looking for a new head if it happened where you couldn't immediately pull over"

Did you have a reason for buying a spring? (I'm glad you had one on hand!) BTW call Pelican, it should be under warranty.

Now I'll add my 2-cents here.
I believe the OM603 alternators with the lack of a clutch create a stress in the tensioner when the engine is shut off, as you now the diesel stops dead in its tracks when fuel is shut off by the vacuum actuation process when the key is turned to the OFF position.
This abrupt action causes the entire engine to lurch and that also imparts energy into the motor mounts, which also are the most short lived things in the OM603 etc. as the gel dries out or leaks out, another well-known high maintenance item.

In the the OM606 and later diesels a new device was implemented which has an OAP clutch with a ratcheting action in the hub of the alternator pulley, allowing the reverse pulling effect to turn the pulley without imparting any energy into the serp belt and therefore the tensioner.
I learned this long ago and have replaced the alternator in my '87 300D with the alternator from a 606. There is at least one discussion on doing this and why it is worthwhile. Here's my post on that subject

DDH

So why was the spring replaced? Did you rebuild the tensioner assembly while you were in there, or just slap a spring in it? The stock alternator will not break the spring. The only time the spring gets stressed is when the tensioner shock wears out and allows the pivot to rapidly vibrate. It destroys the pivot bearing, idler pulley bearing, tensioner spring, and if left unchecked can let the belt off or crack the front timing cover.

Thanks for the info. Like I had mentioned, once I noticed the pulley squeaking I figured why not replace everything involved with that component? So I replaced the pulley, the pulley lever, the tensioner shock, and the tensioner spring.

Did you transfer the spacer washer from the old pivot lever to the new one? If not, fix it immediately or you risk breaking the timing cover.