Is this a transfer box / differential unit? Or is it an automatic drive unit remote, but next to / part of the engine?
If it is a transfer box / diff unit they operate by use of a gear for forwards, and a chain for reverse, normally. A chain, even in the best of times, has a bit of slack in it. So for reverse it can move slightly on selection, which might be enough to gain selection of the drive. A gear, on the other hand is much less likely to have play in it. Thus it will not allow a small movement to facilitate the selection, as is the case with reverse. All this being so, it might be as simple as the idling revs being just to high for the unit to be unloaded, and thus able to engage the forward gear.
The same thing can happen with varomatic drive. If there is a transfer box and the drive belt is rotating to fast, it will be throwing the bob weights out and attempting to select drive. That usually results in no ability to select any drive with the engine running. It can create a nasty gear clashing noise.
If the engine is like the Lombardi diesel, it sounds like a very slow tickover adjusted correctly, like a mini digger, or dump truck. So it might be worth knocking the tick-over as slow as it can go. If that is successful, then you may need to re adjust your throttle cable so it has minimal slack, or you might find that top speed is unavailable for the given pedal movement. It is not uncommon to find these cars with the throttle cable acting as the adjuster, rather than the carb/injector stop, and the tick over to be to fast as a result, or because the last person did not know how to adjust the carb/injector for best performance.
