I asked this question at many dealers and the 1(800)# Toyota before making the choice of Hybrid or not. Not one source had THE specific answer but I pooled the feedback and this is what I understand>>>
The electric motors are sealed and water resistant. So mud etc would not be the issue.
The rear electric motor
is not full time.
The HiHy is not Full time 4WD. Where as the Non Hybrid H/L is Full time at 50/50 front/rear. It is an "on demand" drive mechanism. The HyHi is a front wheel drive Bias design. VDIM determins when the rear wheels get traction. And the
rear wheels don't get any supplemental
engine torque,it is strictly electric power driven. Only the front wheels get a combination of both engine and electric power as needed. Toyota is saying that
extreme load to the rear wheels in an "Off Road" environment would exceed the limits of torque on the rear electric motor. But no one will say what "Off road" is defined as.
So my interpretation is that yu CAN go on gravel but not on extreme grading where the rear wheels would have to take over or sustain the ladened weight completely, momentarily or otherwise.
All this because the
rear wheels are only Electric driven with a low torque rating designed
only to supplement the front drive system.
It cannot handle the full ladened weight of the HiHy.
LT