Thanks for the help. After much research and checking of part numbers and specifications I eventually tracked the part down.
I didn't even bother going to Jeep (pulleys close to $200 ea last time I asked and no availability).
At Mercedes, the OE part number led me to find one unit in Singapore and some in Germany (3 weeks wait!) at $120 ea.
The nuline catalogue has the correct part 'ep253' listed yet it's discontinued and unavailable.
Finally (after Repco, Coventrys and Veale couldn't help) Auto-one were able to order me the Dayco part 89042 (I had to join their website to access the catalogue) from Wesfil and although it's not identical, it has the correct tensioner bearing, the same OD (70mm), width (28mm) and ID (8mm). $60 ea
and I just have to change the bolt length. (shorter)
Good news on the alternator as well - bearings can be replaced (standard Bosch bearings) and the special 'Overrun pulley' is available (ex East). $350 all up. Lucky for me it still functions perfectly well because the repair shop don't have a suitable bench mount in order to test it.
While I was there and noticed diesel leaking from the HP pump, so I sourced and replaced the 'o'rings and special washers. Mercedes had heaps (10) in stock and the spare parts guy was actually quite helpful.
I think as long as you go with the right part number it's like you're attempting to speak their language and they forgive you for having a Jeep.
Hopefully this info can help others with squeaky serpentine belt issues. Thanks again for the help.