After sitting in driveway, back wheels have completely locked up.

It is easier to change the whole half shaft than it is to change just the CV joint.

All wheel drive is just that-- All wheel drive-- not to be confused with 4 wheel drive . The difference is with AWD there is a 3rd diff in the transmission that allows the front and rear wheels to operate at different road speeds . When 4wd is selected that 3rd diff is locked and so both front and rear wheels drive at the same speed regardless of ups and downs in the road surface . This results in a torque wind up in the drive train and places load on "U" joints --splines --CV joints and gear teeth . The grinding on turning corners will be the CV joints trying not to flex because of the torque wind up . How to prevent this problem=== Do not use 4wd on hard road surfaces === as the tyres cannot slip to relieve the torque Restrict the 4wd use to snow , mud and loose surfaces TO fix the problem unselect 4wd drive and reverse several yards to release the pressure in the drive train and the actuator has a chance to pull the gear out of lock up. If that doesn't work check that the actuator is actually working

You have front wheel drive & CV Joints,
not U-Joints ,is that not correct

CV Drive Axles in front wheel drive vehicles
are just replaced as a complete axle assy

could be cv joint but jack up wheels and move tire up down and left right up down if play check ball joints /anti-sway bar bushings--------if play left to right than check idler arms and steering system /and anti sway bar bushings

well now,,,il tell you a storie,,,better sit down?
the cv joints are the joints in a drive train that for use of a better word "bend the drive shaft" so it dont brake when you turn a corner,,,ie:parking! inside the cv joint is held on the drive shaft be some splines in the drive shaft and a spring clip holds it all together there is an "inner bell" it has normaly 5 slots in it that have big "ball bearings" in them that slid around up and down these slots on the out side of the cv joint is the outer "bell" this also has slots that the ball bearings fit into so the drive trough the cv is compleat,,,when you turn the wheels the cv joints slids round on these ball bearings twisting round but the drive from the engine is still transmited to the road wheeles trough the cv
when this cv joint starts to were out the ball bearings are pitted and not ball shaped any more and the slots they run in are not smooth that to gets pitted what happens is the cv starts to "crack" under load,,,and vibrate when turning or being driven you drive down the road at 50 mph and the car feels like its driving over stones in the road its vidrating off the cv joint then when you turn the wheel on full lock to pull away you here a very big cracking or clicking sound, like,,,,,click click click, and the more power you put on the more they click very soon people start turning around and look at you? oh yes they here it to!
the vibration that comes off the cv will reck the transmission becouse of the wipping of the drive shafts,,,, to test your cv joints to see if there good or bad one's put the car on full lock and pull away,, if there is a cracking sound or a klicking sound there shot away and you need new ones fitted
now then,,,storie over,,, its time for sleepy sleeps

Loud popping/grinding most noticeably heard when turning is normally a sign of a bad CV joint. Look under the van where the axle half-shafts come out of the transaxle and connect to the backside of the wheel assembly. Are the rubber boots on the axles torn or missing?
If you hear the popping only in left turns, your right CV joint is bad, If you hear the popping only in right turns, your left CV joint is bad.

inner cv joints are the joints on the inner half of the axle shaftswhere they go into the transmission. sometimes the easiest thing to do is replace the axle shafts

sounds like your front bearing are locked up.  This happened to me once in a van that had been sitting awhile.  

sounds like you might have a broke axel or cv joint