[Help - Can't get 3rd in Low ratio in 2003 TD5 Defender 90]

Both boxes have correct grade oil as per RAVE (correct for MY of vehicle).

[TD5 Brake carrier bolt size]

M12 x 45

[axles]

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...

[Chassis Issues]

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