You might be able to change the input gear to the transfer case, the newer one with the longer splined section; it is a fix that has got many people out of a similar situation as yours.
The old gears and shaft only had a small area of splines, LR realised there was not enough spline surface, so they made the splined areas longer on both the shaft and input gear.
You can easily change the gear for the sake of 10 bolts on the back of the transfer case and wiggle out the old gear, fitting the newer one will allow part of the shaft that is unworn act on the new gear, hey presto, with careful driving it would last you long enough until you can get a new mainbox.
Check before you buy anything though, no point spending 45quid if this has already been done.