The seam sealer on the drip rail is cracking letting water in between that and the body. The drip rail is just spot welded on most of the way to the roof.
Ac
I added one years ago wasn't to bad. The three holes in firewall already have a dimple for location. Just try and get all parts from same year and options as yours because was several different wiring harness and ac cards used.
front dif removal
When you remove the nuts holding the front half-shafts, take a punch and knock the studs out. This will make removal easier. Then it is only 3 bolts other than the drive shaft bolts to remove the front differential. Hope this helps!