Drain it and refill using 2 qts of Lucas trans fix. I bought a tired 96 F350 diesel to plow my camp road with this winter. The trans fluid was burnt and black. I never got around to changing it before a family member got it stuck because it was slipping so bad it wouldn't turn a tire on ice. I pulled it to the garage and changed the trans oil. I added 2 qts of lucas trans fix and have been plowing with it ever since.

 

I think we might have to try that, I think he's having trouble getting the money together to rebuild.