Hey Chris,
It may be irrevent, but the 4L80 in my duelly starting whinning in low gear about 5 yrs ago. The guy that builds my PG's told me it was the plantatries, he also said don't worry about it drive it until broke, because the rebuild would cost about $1600.00. I have done just that, pulling the inclosed trailer, open trailer, etc. and have put 40,000/50,000 miles on it since it started whinning, and it still works as good as ever.
Zip.