Always check fluid level with any transmission issue, of course you only found out this was involving the transmission overtime, so you didn't understand this was a trans issue first so gentle sin not checking the level. I live on gravel as well, you would know if there was any leaks. Of course leaky valve body is something very different, it doesn't leak anywhere visible, it just pushes fluid away from the spots it is needed, it stays inside the transmission. But at least for a while, check the transmission level maybe every fill up, leave car running and car in neutral, yes with parking break on. Neutral spins the fluid, park does not spin it, to get an accurate warm level reading I think you need it in neutral. Especially after some shop services a transmission, you really need to check the fluid a couple times when warm.
If these symptoms come back anytime soon my guess is you need to rebuild VB or get a new ASAP.