Steering weirdness

[wiscburb]

OK, so I've got this bizarre steering problem I'm trying to diagnose in my 89 Suburban. On the highway it rides down the road happy as a clam, steering is straight and nice and tight. Once I get to city streets, though, things start getting funny. Sometimes (not always), when I enter a turn, the steering works great until the steering wheel gets to about 90 degrees -- then there's a sudden lurch in the direction of the turn.

When I had first bought the truck, I drove it on a rutted dirt road, and ended up zig-zagging down the road, barely under control, due to this problem.

A couple of other things which may or may not be factors

1. When I lock the hubs in front (selector in 2wd) and go around a corner, the front tires bind up and the front end hops around. I'm thinking maybe this is an unrelated U-joint problem

2. In messing around with it to try to figure it out, I discovered that if I have my front hubs unlocked, but shift into 4HI, the steering gets crazy even if I'm driving in a straight line. This happens DURING the shift, but is fine once I'm in 4HI. i.e. if I hold a little back pressure on the 4wd shifter, the steering goes crazy.

Any ideas? I'M going a little crazy myself with this issue...

Thanks!