I just picked up a 4x4 FT (Full Time) LX 450, it is the transmission that is giving me problems (or that is my guess), when I start from a Stop the car moves like it is so heavy and once when I accelerate to 70 mph it become lite again, I think the gears are not shifting quickly enough, I changed the transmission oil it helped a bit at times it becomes ok at times very heavy but as soon as I hit 70 it becomes ok.
Note : i have to hit the gas to take the revs up to the 3000 RPM mark to change the gear if they are stuck.........