Unimog Club UK

Hi Tom, welcome to the forum.

The problem with your gears could be that the selector cover has been taken off and not been put back on properly, they are quite tricky, however Moose will know more on the subject than me. It could also be that you are not pushing it completely over the gate, sometimes the gate is like a double step, so the gear lever sit's on the gate, if the gear stick doesnt wobble between the 3-4/5-6 gates then i'd assume this to be the case. So a good knock to the side might well be in order.
You can get workshop manuals from the Unimog museum, being a 64 yours will be a 411b (originally 32ps), you may also be able to get an operators manual.
I would change all fluids and filters, including (especially) brake fluids, a good upgrade is to take the master and slave cylinders off and send them of to be lined with a stainless steel lining.
Depending on how much it's been used recently it may also be a good idea to change the flexi lines on the brakes, these have a bad habit of furring up if the mog's not been used or the brake fluid hasn't been changed in a while.
The 411 is a good purchase, what are your plans for it?

Cheers, Brian