What are these weights on the cross member?

[rhondas]

On my 03 Pathfinder under the transmission there are two 3X4 weights welded in-the cross over beam, what are they for? If this photo comes out, it's in the furthest cross over bar towards the front, one on each side?

[EdBwoy]

I can't quite clearly make out what you are referring to, and don't have my vehicle to compare, but my guess on the purpose most of those random weights on structural and suspension members is for NVH.
On the extreme end, visualize holding a tuning fork or long hollow steel tube at one end and smacking it against a hard object. It will keep vibrating and might even be audible for a while.
Grab it somewhere in the middle and the vibrations stop and the sound is dampened. I picture that these inserts are usually attached to vehicle components to absorb this energy.

[Hawairish]

Man, that is a super-clean undercarriage!

What I think you're referring to (same as EdBwoy) are the dampers on the transmission crossmember. There are many like it all over the vehicle, in particular the rear traction bars and rear axle. As Ed described, they are to control NVH (noise, vibration, harshness).

[rhondas]

I think your right about the being for vibration etc, but it sure does add a lot of extra weight to the truck.