I hope this is relevant to your particular model of transit, but I had a similar problem with my 2002 t280 front wheel drive 2.0tdi and managed to fix this myself. Start by leaving the gears in neutral. From memory, the gear selector is mounted on the top rear of the gearbox housing and has two big cables attached to it. Unclip the cables and remove from the knuckles they are attached to, then unbolt the complete mechanism they were attached to and pull it upwards and out of the gearbox. I found that it was a design fault that allowed water to ingress and had caused corrosion. Once out, clean it up and using wd40 to help you manipulate the mechanism, especially the part the goes up and down, until it all starts to loosen up. Then liberally spread grease over everything and work it in so you should notice a vast improvement. Refit the part back into gearbox and this should sort out your problem. It's a bit fiddly but fairly easy and should take less than an hour to do.
Rupert
May 2018
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