4x4 engaging by itself

[Alexxx]

I have a 2002 Hilux 2.5D crew cab pick up, UK spec. All has been well until I took it to have the AC serviced at a Toyota servicing centre. When they gave it back there was a clicking noice from the passenger footwell which seemed like a relay to me. The garage told me to take it away and see if it disappeared and thought it might be a valve related to the AC system. As I drove onto the road there was a violent thump which repeated itself a moment later. I drove straight back to the garage (having only covered about quarter of a mile). They unplugged the transmission relay, so I could drive the truck, and ordered a new one. This was fitted a week later and given back to me. The fault was still there. As I understand it, the transmission is trying to engage four wheel drive by itself hence the horrible noises and juddering.

I decided to order a new transmission computer (believing there may have been a power spike during the AC servicing, unlikely, but I couldn't think of anything else to try) and have had that fitted at the garage who usually do the servicing. They tested it for five miles and all seemed fine. I drove it home, no problem. Today, two days after picking it up, the problem is back, as bad as ever. The transmission relay is clicking, the green light on the dash, signifying the 4x4 is engaged, flickers and the thumping from the transmission along with associated nasty noises are all present. Selecting neutral and coasting doesn't stop this happening either. I only had a penknife and eventually managed to remove the trim to get to the relay and unplug it so I could get home without wrecking the transmission.

Does anyone have any ideas what this could be? The Toyota garage don't know what to do and I am left with a 4x2 on what is otherwise a very tidy vehicle.

thanks