If the driveshafts were too short, what sort of a garage would fit a part that doesn't look right would be my first question!!!!
When I spoke to a guy at car parts direct, he said there are 6 different sorts of driveshaft fitted to the JTD's. On the 20v's, the driveshafts are the same both sides, it's just the inner cups( I still don't know the proper name
) have different lengths as the wheel on the passenger side is alot closer to the gearbox having a short one and the drivers side is alot longer as it has to go across the back of the engine/sump to the drivers side.
Even re-con units normally have a 2 year warranty(mine do), so I don't quite understand how they can fail unless they were fitted incorrectly, the rubber boots have split letting water in or they weren't 'up to scratch' from the re-con company.
I can only speak about my own experience as replacing the driveshafts and it didn't make alot of difference, it's only cup change that completely resolved the problem as it then allows the spider of the driveshaft to fit really snug into the cup and not allow any or very little movement resulting in the vibrations. This might have purely been because the grease ran dry resulting in the heat build up and quick wearing of the driveshaft/cup. I can only assume something like this happened to yours to fail that quickly?! It's probably worth checking the engine mounts too though as if the engine isn't being stabilized effectively, it could possibly adjust the angle the driveshafts are sat at and contribute to early failure, maybe?!
I got inner cups from the dealer £112 for re-con, £200 for genuine parts.