I have a 2000 F-150 with about 89,000 miles. When I engage 4WD the front axle locks as usual. However, when I turn the wheel more than about 50% in either direction, the front of the truck starts to bounce, or jump. The more the wheel is turned, the worse it gets. This happens on smooth surfaces or dirt/snow. Any ideas what might be causing this? Thanks!

 

it doesn't sound like it's slick enough from your post. snow can be pretty tacky to the point where it will buck. Dry dirt will definitely cause some issues. find some mud or ice and see what it does.

 

I tried it again this weekend in soft sand and it still bucks like crazy when I turn the wheel. Would bad U-joints cause this?

 

It does this because when you engage 4wd, each wheel is spinning at the same speed. So when you turn, the inside wheel will 'skip' to keep up with the outside wheel. (When I say this, I assume you know that when turning, the inside wheel spins slower than the outside wheel). When I bought my 4wd, I did this on pavement to ensure 4wd worked properly.
So the sharper you turn the steering wheel, the more the inside tire has to catch up with the outside tire causing it to get worse the harder you turn.
Hope this helps.

 

You have differentials in both axles that let inside and outside wheels on that axle turn at different speeds in a turn. The Ring gear between them turns at the average speed .... not as fast as that outside tire, but faster than the inner side tire. It turns the drive shaft at some speed based upon gear ratio.

There is no differential in most of these 4WD truck's transfer cases and In 4WD, the front and rear driveshafts are locked to turn the same speed.

In a turn, the rear axle will track inside the front axle scribing a tighter radius (just like your trailer tracks inside your truck in a turn) .... and closer to the center of that arc.

The outer rear tire will track inside the outer front tire and the inner rear tire will track inside the inner front tire.
Closer to the center means shorter distance, yet both are getting to far end in same time ...
... thus the rear axle itself is wanting to turn slower than the front but is forced to turn the same speed. This is "pushing" the front axle further than it's ready to travel, so every now and then as the driveline is "bound up", a front tire (usually inside because truck's mass is pushed over onto outer side front) looses traction and "skips" forwards to relieve tension.

Imagine you and your girl walk exact same speed, her right behind you. She follows around corners walking in your foot steps, she stays back there. If she walks a little inside, cutting the corner, taking the shortest distance .... she catches you or bumps you ... or you skip ahead a step as needed.