Brienobrien,
Scudder is right. I will also add, from direct experience in owning two MGBs (B and GT) that there is also a vacuum operated switch on the intake (near firewall) that can also fail. $3k is way too much. You can get a recon unit and put it in yourself from Moss Motors (check the Internet). Do NOT use it in this condition. The O/D is electro-magnetically activated by both solenoid and electro-magnets. “Jumping” in and out of O/D will cause it grief-- to say the least. Also, check all grounds, etc. Remeber, Lucas, the prince of darkness.
Cheers,
Matt