Well first gear is nothing more than the rear clutches and over running clutch w/o a low band apply valve body. If the rear clutch was bad you wouldn't have 2nd or 3rd either, the car wouldn't move so this does sound like an over running clutch problem. Take it out and apart before it does more damage.

"Follow me the wise man said, but he walked behind"


'92 D250 Club Cab CTD, 47RH conversion, pump tweaks, injectors, rear disc and hydroboost conversion.
'74 W200 Crew Cab 360, NV4500, D44, D60 and NP205 divorced transfer case. Rear disc and hydroboost conversion.
2019 1500 Long Horn Crew Cab 4WD, 5.7 Hemi.