I have a 2000, Nissan, Frontier, XE, 2WD, pickup. I let a friend use my truck while I was deployed overseas; when I got back I noticed a clicking/popping sound in the rear end when I would slowly turn into parking spots. It gradually got worse, starting to whine when I would let off the gas on the highway and decellerate. The other day I was leaving base and as I turned the corner there was a loud pop. I gave it gas and I didn't go anywhere. The drive shaft now spins freely into the rear axle in all gears including reverse. Sometimes it will act like it wants to catch, but alls it will do is shake the truck. Any ideas? Your help would be greatly appreciated!

I think the rear differential may have been run with insufficiant lubrication. Likely, the gear oil leaked out of either the axle seals or the pinion seal, or both, until there was not enough to keep the gears lubed.
I would suggest calling around to salvage yards for the replacement, or try finding a shop to do it for you.
For most DIYer's, this is a fairly big job.

Sourced a 3rd. Member for $250 at a local junk yard and threw it in. Fairly easy job, about an hour. Truck back on the road, but there is a predominant rear end shimmy now. Feels just like a bent rim, but its not... Believe me! So far I have changed the transmission, transmission mount, all 3 u-joints, center support bearing, had the driveshaft balanced, shocks, rims & tires, machined drums, both axle shafts, bearings, seals and shims... I had the thrust angle and pinion angles all checked out... Still there exactly as before. Buying a complete rear end assembly from Nissan for $1,221. Will be throwing that in after I get back form this deployment.

When vehicles with limited slip differentials sit idle for long periods of time/or/ when the differential fluid loses it lubrication quality the rear clutches will grab when the differential gets hot and makes a tight turn. That is when you will get the popping sound. Go to your local GM dealer and purchase a container of limited lubrication additive. It's whale oil . This should eliminate your problem without a teardown.
Tune Bethune

Uautoknow, I agree with your assessment. Because of the no. of views on this thread over time, I'm going to add a little information, based on the Xterra/Frontiers I've worked on.

Rear Differential (C200). 75W-90 Mobil 1 Synthetic GL-5 LS Gear Lube, 3.4 pints (54.4 ounces).

Fill Plug (14mm Hex Bit): 26 lb.-ft. Apply RTV, Three Bond Adhesive, or Teflon tape to fill plug.

Drain Plug (10mm Hex Bit): 20 lb.-ft.

The Mobil 1 product comes with limited slip additive and you shouldn't need to add any more. However, it does not hurt if you do--add any additive first and adjust the amount of gear oil accordingly.