Those adapters are not the best things around, we used to service them on our school bus fleet all the time. We had a whole box of gears and drive shafts for them at one time. Pull it out and see if the input turns the output. The gears are just pressed on the shafts and when a tooth breaks off, the whole insides come apart. I believe the input side has the removable drive, sort a cable looking piece with a couple drive ears on it. These used to break all the time on us. If the adapter seems right, then one or both gears in the trans is worn or slipping. The adapters can be serviced, just remove the screws and clean out he old grease, just be careful of the gasket. If the housing is turning on the mounting shaft, I think you can tighten them up from the inside with a little ball peen hammer.
Joe

1937 Chevrolet Inline-powered Streetrod