Are they both Santa Rosa? You need a different species for a pollinizar.

start at the tip and cut, if dead work your way down.

No the other one is a beauty. I'm more concerned with the lack of foliage than fruit. I wasn't planning on fruit this year anyway.

Because this looks like a sucker branch to me anyway. None of last year's branches have anything

Definitely keep that green shoot, if the rest of the branches are dead then that growth will become your new leader.
Just start pruning back until you see living wood.

Well let me contrast that photo with what the sister tree looks like

Sure, maybe I should do a little weeding (don't judge me!) but the difference is pretty dramatic. If the other one

has nothing by now, is there really a chance there's any life in the remaining branches? And if I cut them all off then I have a tree with one branch.

Also, I thank both of you for your help. I'm just a little worried for this little tree and if I have to replace it the lady with the vacant lot nursery across the street has one left from Monrovia nursery so if I need to replace it sooner might be better, even in the warm weather

Something is up with that tree. Cut a couple of the other branches off. If they are dead cut them all off. Personally I always avoid Monrovia fruit trees/bushes. They seem lower quality than Dave Wilson or Four Winds. Both varieties of plums you mention are self fertile you won't need a pollenzier.

You might try a half strength dose of a high nitrogen fertilizer like miracle grow. I say half strength because it's super easy to burn the roots with a high nitrogen fertilizer. Aside from that, I would just wait. The tree looks great to me, just still mostly dormant.