So this xj I'm trading my f100 for has the dreaded "no reverse" problem. And I'm not sure if I should do a flush and service or start looking for a working trans. I guess I need to trade and disconnect the tcu and see if it still goes into gears. Atleast that's what everyone is saying I should start at.
Drain what you can at the bottom pan plug. Add a blue can of Trans-X, 1 quart, top off with Dex-III only, then with the parking brake on and tire blocks in place, place it in reverse, and let it idle for 30 minutes.
This will clean the valve body, seals, O'rings, etc.
Then drain again, and refill with Dex -III ONLY!!!
Then drive it a while. Then do one last drain and fill with Dex-III.
If there is still no reverse, you could try a new valve body, but that is not likely to fix it, as the seal or clutch plates that go out and usually cause this are not in the valve body!!!!!
But it could just be a stuck piston, vanish problem, and the method above may work.
I kept 2 of mine going for 4-5 years before it finally stopped completely on the 87.
Long term I would start looking for a spare.