Had a deal in 2019 that sounds similar to that problem - a couple of the snap rings had been improperly installed in a previous rebuild, and the internals were moving fore & aft.
Basically, the trans was eating itself; went from "what the hell is that" to a horrible grinding racket very quickly.
Had to replace countershaft, synchro rings, bearings, and snap rings.
Brewer's had everything for about $200 and shipped quickly.