2006 48re.
was experienceing some of the 1-2 shuttle shift, then pulling my trailer started to get some major slipping in what was first, wouldn't shift to second would just pop outta gear and rev until i let up then it would grab, give it some throttle and pops out and revs again, on a hill became very very pronounced only at slow speeds.
So bought a borg warner solenoid and transducer. Dropped the pan, fluid was very black, about 1/8inch of sludge on the magnet, changed the filter also. Put it back together and filled it up with fluid. dropped the rear cooler line to try and get it to flush some and started it up, nothing came out after 10 sec......shut it down and scratched my head. hooked the cooler line back up and fired it up, threw a code, low transmission fluid pressure, couldn't clear it.
went for a test drive and right off the bat knew something wasn't right. Almost felt like a second gear start but was just reving like crazy to get going, if i put my foot into it, it would go but not hook up at all. Couldn't hear or feel any shift points at all felt like i was on a snowmobile for how high it was reving and no shifting.
tried to manually bring it to first gear, no change, no change in second either doing exactly the same.
check engine light is on can't clear it.
Did something get plugged?
why would nothing come out of the cooler line? was it cause the trans was cold?
what is my next step?
any help please

 

If first gear is gone, all forward gears are too. This is the rear clutch. If you kill it you may have to replace the hard parts too and that will add $500-700 so be careful.
Time for a complete rebuild.

 

But why would changing the pressure solenoid and transducer and fluid change that so bad? The sudden change makes me think there is a problem with the part or something is plugged.

 

i think j-fox is on the right track. it sounds like your clutches are gone. you could try to flush it but what you describe seems pretty close to a meltdown. if you have a programmer you could try to reflash your system and see of that helps. has your new fluid changed colours?

 

That is not easy to dignose from here!
Not intending to insult, but maybe the filter is loose?

The fact it had so much in the pan, says open it up there are clutches shot. you are not going to fix this trans with fluid or additives.