I've had the same problem (mine went in after a struggle but wouldn't release without tapping the lever with a hammer!) and just fixed it last week. I believe it's a common problem on 40.10's. Yours should be the same as on my WM. Basically it just gets too tight for the springs.
Take the mechanism off the axle, 4 bolts (2 long, 2 short) after disconnecting the pull and return springs. It works via a peg which locates in slots in a collar located on splines on the half shaft. There's a roll pin on the lever; if you were to drift it into the housing it should be possible to release the cross-shaft and get the pin out again but I didn't fancy trying it just in case! However, between the lever and the housing you'll see a felt washer (when the muck's cleaned off). The lever was almost solid but after soaking the felt washer in WD40 and leaving in the vice overnight it was much better. Another overnight soaking and working the lever back and to and it was nice and free. Then I soaked some '3-in-1' oil into it. Put it all back together and check the return spring can pull the lever back again and that the peg is well clear of the slot when in the 'off' position. (Jack up the left hand wheel and turn it by hand). While I was at it I lubricated the cable and the clevis pin on the other end under the cab floor. I suppose you could do the same without removing the assembly just by cleaning the felt and oiling it in situ but I wanted to see how it worked! A drop of oil now and again is now on my regular service list!
Apparently the RH shaft is driven all the time, the LH one can be locked with the peg engaging one of several slots in a collar which is splined on to the LH shaft (the LH shaft has two sets of splines, one at the end for the diff and another about 6" along the shaft for this lock. So it's not so much a difflock as a halfshaft lock, but the effect is the same anyway!
Hope this works for you too!