This happened to my truck at 108k backing out of the drive nice and slowly, small hill.
Best I have found is the reverse sprag clutch broke, it is a one way clutch buried in the end of the trans meaning it needs to be torn down 100% to check and replace.
As vansnxtweek mentioned check if you have engine braking in L1. Just get up to 30 or so and drop her in. If its like your in neutral and the rpms dont raise its most likely toast.
Upside is I put 10k on my truck like this until i finally get a rebuild in. I suggest getting a warranty with your next trans, I waited to get my trans in and burned up my 3 month warranty while it was sitting in the garage. Turned out my rebuild was not really rebuilt. It works but it has really low pressure. Im running a stage 3 shift kit and tweaked bands to get a slightly harder than stock feel.
If you have to wait for a trans just be creative when you park. I was able to survive a winter with out reverse, probably why I didn't want to change it out.
99 x-cab 4x4 4.0l auto
mods, cranked t-bars, silencer delete, k&n drop in, Rebuilt trans ,Trans Go stage 3 shift kit, 3200stall Fuddle Racing Torque Converter,
VANIR 63mm TB, Roush split rear exit exhuast, E-Fan, JBA Headers
Auto up/down both windows, OHC, EATC (soon to be modded)
03 Crown Victoria Police Interceptor