91 cummins 350 automatic transmission stopped going forward but has 1st and reverse
 

Please help me. I was driving up my steep driveway and my truck just stopped going forward. It revs up still and has 1st gear and reverse. I dropped the pan and did not see any metal in it. I don't know what to do

You either have a front band/servo issue or something is broke in the planetary gears.
You can try adjusting the front band, beyond that pull the pan and make sure the anvil and strut are in place on the front band. If that is all good then it's most likely time to pull it out.

I've had a few Chrysler rear wheel drive trannys apart.
Just by what you stated in you're post.
If you still have "manual 1" & "reverse" but, it revs up & don't pull in "breakaway 1st" (that's what my Chrysler transmission instructor referred to 1st gear in the drive position half a lifetime ago)
i would be looking at the "overrunning clutch/ low roller clutch"
Breakaway 1st is: rear clutch applied & overrunning clutch holding.
Manual 1st is: rear clutch applied & rear band+ overrunning clutch is "partially" holding.
Reverse is: front clutch applied & rear band holding.
It's not unheard of to break the overrunning clutch & not see any debris in the pan initially.
The rear band applied in manual low is what gives you engine braking in manual low.
Either way the trans is coming out. Hopefully the only hard parts damaged is the overrunning clutch.
Just my $0.02
Good used 4 pinion planetarys & any output shaft with a bolt on yoke are hard to come by here. Last year i had to fix my 90 D350 TUG trans & i got lucky! I had a A727 Mitsubishi FUSO diesel trans core & driveshaft laying around for about 15 years.
Best of luck, HMFG

You are correct. I didn't read that as he was pulling the shifter into manual 1st. Since he didn't specifically say that, I assumed he was in another gear when it quit moving.