What could cause metal on metal grinding at 40 mph and above on '96 Pathfinder?

check the "u" joints for looseness and wear especially the front drive shaft . check sliding splines . The vibration is a harmonic vibration normally associated with worn "U" joints --bent or out of balanced ---worn /seizing slip joints or being excited by a failing drive thrust bearing in the diff. ( big bearing directly behind the pinion gear that takes all the load on drive) check the magnet on the diff drain plug for metal bits.

check your engine mount for play as a weak mount causes exhaust to vibrate could also be a heatshield not fitted properly

I would start with the easiest and get your wheel balance checked.

Replace the wheel bearings

sounds like a busted strutt try jacking it up and shake the rear wheel if goes sides to side easily its strutt up and down its a wheel bearing.

Most common cause is the brake pads have worn and you have a metal to metal situation where there is no life left in the pads and they are scraping the rotors. Another cause could be a hung up caliper (you would notice the truck pull to one side or another when you apply the brakes if this is the case). One last possibility is worn out brake bearings that will grind when faulty. Either way, it sounds like you need a brake job that will include pads and rotors.

I don't think any body can help you here.
any mechanic have to drive the car to experiece the problem . Impossible to fix it over the phone.
Take it to mechanic shop to prevent futher damage.

Sounds like your 4x4 lockouts are engaging (only if 4x4) and if 2wd then could be wheel bearings, driveshaft center support bearing. Also could be tire(s) when the metal belt gets exposed it also makes a harsh vibrating sound. Start eliminating some of these and see if you can pinpoint if its coming from the front or rear or center of the truck

where does the sound seem to come from,under the hood or where,if it is under the hood you might want to replace the fuse and turn the AC on,there is a chance the friction clutch plate in the AC compressor is goneand that will be cycling on if your using the defroster

Its most likely time to change those break pads, and shoes, also wouldnt hurt to get an alignment while your at it... hope this helps!