TH400 whining

[BAD1916]

Thought I’d post this up before I yank my trans...
I have a 71 GMC 2500 454(original 402 block was cracked) with a TH400 from a 67 chevelle( don’t know if that matters any). This setup has been running for 4 years and this Is my normal driver. Anyhow, a few weeks ago I left my house and I could hear the trans whining so I pulled over and put it in park. I didn’t hear anything while in park so when I shifted back to drive the noise was gone. I assumed it was a fluke. Fast forward a few weeks and this morning me and my boy are headed out to go fishing, got the kayaks all loaded up and while enroute the whining started again but this time it didn’t stop. I tuned around to head home and when I got stopped at a light the Burb would hardly move. So called AAA and got tow hooks instead of fishing hooks. At home the truck seems to shift fine and goes in and out of gear fine though I have only moved parking spots.Sorry for the long winded post and if you’ve read this far do you have any thoughts on anything to look at before I pull this thing out? Thanks in advance for any help.
Bryce