toyota four wheel drive question

I have a 1997 Toyota 4Runner SR5 V6 Automatic 4x4. The owners manual says that I can lock in to 4H while moving as long as vehicle speeds are below 50MPH. My question is once I am in 4wd do I have to keep the speeds below 50mph?

Yes, otherwise you will shorten the life of your CV joints.

Not sure if you have the same system as the Surfs, but in 4H you can drive at what ever speed you need, we have 4H locked which means that the centre diff is locked and should only be used for slippery surfaces, otherwise the trans will wind up.

See here:

3rd Gen: The 3rd Gen Surf has a multi-mode 4wd system.It may be used in 2wd Hi-ratio, driven by the rear wheels only, 4wd Hi-ratio with an open centre differential. 4wd Hi-ratio with the centre differential locked and 4wd Lo-Ratio with the centre differential locked.H2: Rear wheel drive only: Normal operation, best for less wear & tear on components, best fuel economy & performance.
H4: 4 wheels driven, open centre differential: Press red button on side of 4wd selector. 4 wheel Indicator will illuminate in centre of dash when fully engaged. You may select/deselect this mode at speeds up to 100Kph. May be used anytime extra traction is desired
H4L: 4 wheels driven, centre differential locked: Move 4wd-selector lever fully back (position of button on side not important), 4 wheel Indicator and centre diff light will illuminate in centre of dash when fully engaged. You may select/deselect this mode at speeds up to 5Kph. Must only be used on loose surfaces such as mud, snow etc.
L4L: 4 wheels driven, centre differential locked, Lo-Ratio engaged: 4 wheel Indicator and centre diff light will illuminate in centre of dash when fully engaged. Select neutral on main gearbox, move 4wd selector-lever fully back, push to the right and then forwards.