dont see any retaining clips holding the u joint?
do they just press out? what keeps them in place?

The retaining clip is behind the yoke ears, nearer the u-joint center. Dig around with an old screwdriver, you'll find them. Squirt the whole deal down with WD40 or a good penetrating oil, you'll thank yourself later.

Mine are clicking on the front of my 96' 2500 4x4 Ram also. Looking at them they don't look like there is any easy way to do them. Is it possible to get them out & new ones in without taking the whole hub assy apart? I was trying the other day & was having a hard time just trying to get the retaining clips out.

As far as I know you have to remove the hub, and them pop the axle shaft out. Usually when a U joint is worn out the ball joints are dead also, so it's a good time to replace those too.

This link may help:
http://dodgeforum.com/forum/dodge-cummins-diesel/196964-diy-ball-joint-replacement.html

This should be in the truck area