I went ahead and ordered a new clutch assembly and coupling. I'm most suspicious of the clutch, I guess, so I figure I'll go ahead and replace it while I have the whole thing torn apart. Time to replace the coupling before it fails, too. If I'm still having issues, I'll try the pump next. That's easy enough to replace without a major tear down.
I took a look at the drive block and it looks good.
I'll let you know how I made out in a couple days. Thanks for the advice!