Found this on another thread. Sounds fairly straightforward...?
"The transfer box should physically bolt up...I just did an LT230 conversion to a Classic Range Rover using a D2 box with no problems. How far from bolting up are they? Has the input gear from the transfer box been out for inspection? It can be put in backwards and that will cause the two units not to come together close enough to bolt up...also, check that there are dowel pins in all the right places and that you don't have two butting together. To fit the D2 box to the Defender you will have to...change the front output flange, block off unneeded sensor holes...they are mostly 14x1.25mm thread size and you can use oil drain plugs that size for the purpose. The rear output housing will have to be swapped as the D2 casting is not machined for the speedo drive. The high/low shift lever is different, too...swap the housing so the linkage will fit. When you bolt the two units together, mind where the bolts go...make sure that the 2 short bolts fit back into the holes they came from...if you mix them up and put the long bolts in the short holes, you will break the gears and lock up the box. The D2 box also utilizes a shift solenoid which your original box did not have and your truck has no way to actuate...take the cover off, pull the solenoid and reseal the now empty cover. The D2 box can be made to fit."