If you have a tag axle BEHIND the drive , do you still need a landing gear ?
Moose.
(A moose don't have a landing gear as it will take too long to crank it down...)
If your carrier requires straight trucks to have landing gear (a/k/a dolly legs), the answer is yes, you must have it, even if you have a lift axle (tag or pusher) or two drive axles.
Our truck has landing gear but it is rarely used. To protect our truck from damage, forklifts are prohibited from entering the truck. Operators place the freight on the dock floor or just inside the truck and we load it from there with our pallet jack.
On rare occasions, because of the freight shape or weight, we may allow a forklift to enter the truck. Landing gear is then used. Even with two drive axles and eight tires on the ground, the truck is far more stable with landing gear deployed; in fact, rock solid. The landing gear keeps the truck from dipping when the forklift enters. It also provides the ability, at many docks, to align the truck floor surface with the dock surface, giving the forklift driver level surfaces on which to operate. The risks of losing control of the forklift or freight are thus reduced.
Sure, you can say, "I've had forklifts enter my truck without landing gear deployed hundreds of times and have never had a problem." But how often do you want a forklift driver to lose control in your truck, one time in 400, one time in 1,200? And it may not be a matter of losing control. A sharp dip of the truck may result in a deep gouge in your floor when a corner of the freight hits it as the forklift moves in.
Yes, landing gear takes time to deploy, but it is done with safety in mind; the forklift driver's, the freight, and the truck. While our landing gear is rarely used, we would not want to be without it. I would think this would be even more true for D-units, which carry bigger loads that often require a forklift to load them.