my f550 that i sold to my dad it think just lost it trans. it is a 2001 f550 4r100 2wd 73k miles stock cooler. today we pulled a load 6000lbs or less 20 miles trans never got hot 195* backed it in down a hill to unload. when i tried to pull up truck would not move. fuild looks darker then new and has a smell, no codes. i towed to home and all it will do is back up, in drive-1 it will try to catch do then slip and no movement. i have a 6.0 trans cooler that i was going to put on it this week. the funny part is that it shifted great no problems until it stopped. i dont know if the trans has been replaced or if the diode just died.
im 99% sure it is time for a new trans.

 

When mine went back in March, it went quick. The first time it slipped, I wasn't even sure it had, and by the time I turned the radio down, it was fine. 20 miles later, it was gone. We use ours to plow snow, but baby it as much as possible, not slamming through snow banks like a D8 bulldozer like a lot of guys around here do...

 

Sorry to hear that especially with such low mileage.

Here is some info from a guy named Hiram Gutierrez on youtube. I found it helpful. Give this guy a minute it definitely isn't his 1st rodeo.

Here is his quote:
By disconnecting the connector for the solenoid pack, the transmission should default to 4th gear. If you have reverse, your 3rd clutch pack is working fine. It uses 3rd and low reverse clutch packs for reverse. Good reverse means good 3rd. Heavy take off with solenoid pack disconnected, good fourth. This leaves you with only a valve body issue since all your clutch packs are functioning.

So maybe not as bad as you think although the dark fluid doesn't sound good.

 

i will have go to town to look at the video.

 

i dont think the valve body is what is bad. i moved it to day and you can fill it slipping in reverse and no forward. drained a cup of ATF and you can not see though it, black and gritty looking.
guess it is time to learn how to rebuild a 4r100