Help! TC not locking up at all on '94

[Sojourner]

About 7 or 8 months ago (< 10K miles) the TPS was replaced on my '94 CTD due to the typical locked/unlocked action at highway speeds. I am uncertain if this is related, but prior to replacing the TPS, once in a while the torque converter would unlock at highway speeds, illuminating the orange LED on the 'O/D UNLOCK' button. I would simply cycle the button and it would go back into lockup. After replacing the TPS it would still do this same thing once in a while.

I went out of town for five days, parking the truck at the airport. It acted fine prior to parking it for five days, then when I drove home the TC would not lockup. I took it to the dealer, and at first they told me the bushings on the linkage were worn out and they replaced those. I also had them install a new throttle cable at the same time (200K miles on truck). Now they tell me they can't get the voltage from the TPS down to where it should be and they are saying that the next thing 'we need to try' is replace ALL the linkage as well as the 8 month old TPS (they tell me the TPS has 'worn into' the old linkage and will not work with the new). This amounts to about $800 in parts even with my discount. Then the tech tells me, "There is no guarantee this will fix the problem, it could be in the transmission." At the moment the service mgr. who is a good friend is out at a business conference and I am without his counsel.

Help! I can really stand some direction here. All input appreciated. TIA!