On the 340H there is no 4Th/OD when 4wd is used as this solenoid is used to engage the transfercase, Also note that the torque converter has a 3k stall and is not turning the pump much below that, Which the transfercase needs.

Over time and maybe lousy service techs which bend or stress the 4wd lever linkage can keep it from shifting the lever on the trans.

If your 4wd linkage does not move the lever on trans all the way redirecting all the fluid pressure to the transfercase it will blead off to the front section making the transfercase dissengage at low RPM.

This can be an accumulative problem if you dont maintain the A.D.D. vacuum lines to the front diff. (For those of you with ADD of course)

 

It could be the way toyota designed it... I dont have any evidence to back up my claim but I know toyota often puts features on the 4runner to make it easier to crawl. The high kick in of the torque converter in 4 low could be normal.

Also its normal for it to shift at higher rpms and shift a little harder in 4 low. If your shifting into 3rd in 4 low try first gear four high