The RTV-X diesels have a release. You push the brake pedal down firmly, and while you do, the cable that's attached to the brake pedal pulls a release lever on the HST which relieves pressure, easing the shifting effort. They ain't always adjusted perfectly due to normal wear & tear, etc, so if you're having issues with it grab your work shop manual and thoroughly review the cable adjustment procedure. If you get it too tight, it will be a total slug that won't pull in LOW gear much less in high (remember it releases pressure in the HST...).
It works slick as long as everything's adjusted properly.
And therein lies a second "problem". The gears that mesh are cut such that they are very strong for their intended usage, but the downside is that their shift "dogs" (and rings) do NOT let go of each other if there is any pressure on them. If the gears (and dogs/rings) were cut differently to make it easier to shift to/from gear, they wouldn't stay in gear while in operation; which is a whole separate problem.
IMO, ditch the HST crap and put either a real automatic transmission in it, or a manual shift 4 speed with low range. BUT since 96% of Americans (and probably similar worldwide) can't drive a manual, there would likely be a whole lot of clutches being replaced.