I regularly stay in low range in between climbs/descents, and reach speeds up to about 40kph in 4th gear. Though I never use 5th gear in low range. On some tracks it would be a pita constantly stopping to swap between hi/lo. In some situations such as outback tracks, going over 60kph in 4H shouldn't be a problem either. The manual says not to engage 4wd on dry hard surfaces, and speeds over 50 mph (80kph) in 4H are not recommended. It also mentions to engage 4wd at least once a month.
This is what it says on the sun visor sticker on my d22.
"1. (2H): Use this position for normal driving.
2. (4H); Use this position for driving on rough, sandy or snow covered roads. Set free wheeling hubs to "LOCK" when using this position (if equipped).
3. (4L): Use this position only when maximum power and traction (eg. uphill, sandy or deep snow covered roads) are required. When using this position, always set free wheeling hubs to "LOCK" (if equipped). Speeds must not exceed 30 mph (50kph).
Note: Always stop your vehicle before shifting between 4H and 4L positions."