Highly recomend the Ashcroft lockers myself, had a pair fitted to my Disco (before I sold it to go to Steyr products) and they were great. From a design point of view they're a much better product than the ARB, people just seem to go for the ARB "becuase everybody else has them", the design is weaker...
If that section of the chassis is that bad, id be very carefully checking the rest, especially the back curve around/behind the rear springs. Id imagine its all pretty bad, rotten from the inside out... Bad luck I think