Trans

I have a 94 auto and when it shifts into overdrive the thing feels like its missing I drop the exaust thinking it was in the engine with a miss but the engine purrs. It's like the convertor is not locking up. I have a friend with a 92 Ranger with the same trans and his does the same. It's fine if I dont take it out of drive. could it be the maf or speed sensor We both have the 4.0 liter.
Thanks