Re: 88 YJ - 4wd
the owners manuel says:
this position is designed for temporary use of 4-wheel drive for traction in rain, snow, mud, sand or when driving off-road. do not use 4H on dry, paved roads. to engage, shift the t-case from 2H to 4H while the vehicle is moving at any legal speed"
i rarely use 4H- but have a few times in the snow- helped a lot. i shifted into 4H at various speeds.
BUT, NOT 4LOW!
manual says slow the vehicle to 2-3 mph but i say come to a dead stop.