It's a 99' Cignal Melbourne Express but I already know the rear spacing is 135mm.

I wouldn't call a Shimano XT hub "cheaper", it's a very well respected mountain biking hub. I doubt that my wife and I riding casually on a multi-use trail or the C & O Towpath is exerting the kind of forces that freeride mountain biking does on the hub.

Since the rear spacing of the bike is only 135mm the majority of tandem specific hubs are too wide to fit and since the bike is only half a step up from a department store bike so it simply isn't a smart investment to buy a $400 Phil Wood or White Industries 135mm spaced tandem hub to put on a bike I paid $200 for [used]. By that I mean I'd be better served spending the $500ish I'd have spent on just a new rear wheel and put that toward a whole new bike that would probably come with nicer wheels. This entire wheel build (hand built for strength) will cost less than half of one of those hubs and the wheel can go on my mountain bike if/when we upgrade to a nicer tandem.