Anything is possible - i'm not familar with either the Ultima or Durango buggies but i'm sure it can be done, i'm guessing the the 210 gearbox is a modular item, so its a case of then looking at what is needed to mount your shocktower/rear bulkhead & rear wishbones - are such elements part of the gearbox mouldings on the Ultima?
The hardest thing with any such conversion is making the driveshafts work... I put a Losi XX gearbox into the back of my Mardave Cobra - I ended up cutting & spicing together two sets of driveshafts (two losi halves welded together with two cobra driveshafts to make a complete pair)
