Are you sure it's not whining in high range? First thing to do is check your oil level because 9/10 it's always the simple things if the oil level is right you could try adjusting the brake band in the gearbox done by the following: (taken from another page on this forum)
1.Drain transmission oil
2.Unbolt brake cylinder cover(Square with 4 bolts and a round lump in the middle of it on LH side of gearbox near the bottom)
3.Remove spring and replace cover.
4.Tighten large adjusting screw with (22mm??)locknut above cylinder cover until it becomes tight and then slacken off 1 1/3-1 1/2 turns.
5.Tighten brake band.There are 3 screws with (17mm??)locknuts.One on each side of the transmission and one on top under the panel in the floor.Tighten each screw until it just becomes tight and slacken off by 1/3 turn and tighten locknuts.
6.Remove bolt on cover on RH side and you can see the torque mutiplier drum.Check that the drum still turns freely.
7.Reinstall spring in the brake cylinder.
8.Replace cover plate on RH side.
9.Replace transmission oil.
Thats the condensed version from the
manual .They say you can park the tractor facing uphill and save having to drain the transmission but if you leave the oil to settle and put the good stuff back and top up the rest with fresh oil you will have done two jobs at once!
On a JD they made all this self adjusting,apart from that it is the same! If you ruin a gasket you can make a new one out of a corn flakes packet by scribbling on it with pencil to get a graphite coating-I've had to do this a few times as I doubt if any one will have these gaskets in stock on a Saturday morning!!