1999 GMC Jimmy 4wd has become hit and miss to engage

From your description it sounds like the right bearing is failing again.

[quote:b592db02bc="SWilliams"]From your description it sounds like the right bearing is failing again.[/quote:b592db02bc]

Is it the right or left? I jacked up the front wheels and the right tire had no movement where as the the left had some.

Usually you can tel by the way the sound changes when your making turns. You said that the noise increased while turning to the left and disappears when turning right. That usually means the right side bearing is the problem.
The reason has to do with the load on the bearings. As you turn left the right side bearing gets loaded heavier and the steering input also loads it with an angular input. The opposite occurs on a right hand turn, the bearing on the inside of the turn gets unloaded some.

Motion in the bearings isn't good but that usually isn't the cause of noise. Most of the noisy bearings I have dealt with were nice and tight when moved by hand. However the races and ball bearings were damaged, either brinnelling or metal flaking off the surfaces were the problems.

In your situation I would replace both bearing assemblies.

As for the 4X4 problem that is likely a separate issue. Could be a vacuum leak or solenoid problem or if you have the auto 4X4 option the front axle may be failing.