Unimog Shifting Glitch?

So I bought the unimog for the forzathon challenge, and I engine swapped for the MB racing truck engine. Not the Volvo iron knight engine. And I fully upgraded the drivetrain, but the truck shifts like its riding the clutch. I use manual w/ clutch, and have no issues with other vehicles. But after I shift gears and try to accelerate, it will rev high before quickly dropping back down and then accelerating normally. I’ve even switched my entire controller scheme around to where my right thumbstick is the clutch, and I release it as slowly as possible and then accelerate, but it still doesn’t work right. Anybody else encounter the same problem? It’s a shame I’m going to have to buy a different truck in order to not feel like I’m driving with a messed up transmission or clutch.