400 differential

what could cause my rear wheels to not turn independently? It worked when I parked it but now with one rear wheel jacked up it wont turn and if you drive it the rear wheels won"t turn independently. All I did to the rear was change the hytran fluid and the brake disks were stuck from sitting so I unstuck them Thanks for any help

The differential is driven from the transmission output shaft. The ring gear turns the carrier which moves the spider gear shafts #45080DA (end over end) with a spider (bevel) gear #361061R1 on each shaft end driving the side (pinion shaft) gears #361060R1. There is no other connection between the left and right wheels than the side gears, the spider gears and the spider gear shaft.
The possibilities as I see it are:
1}If either side gear was frozen to the carrier it would lock the sequence of gear motion.
2}if the spider gears were frozen to their spider shaft it would lock.
3}if either spider, or side gears were jammed or broken in relationship to each other it would cause the issue.
The major issue is that any of these would be very rare. They would need to have happened when the tractor was in operation if they were breakage, possible before the tractor was put away. The rusting seems also unlikely as rust that solid would be difficult to achieve in even 10 years unless it was in high moisture conditions and not oily.
Forcing the wheels to turn independently by brute force is a bad idea. You have already seen that steering forces and tractive forces including wheel braking do not break it loose. More of that would break expensive parts (that are not already broken.
Though not a desired recommendation, I think the top will need to come off. It is heavy, it pulls the shifter forks off with it, and is usually stuck down hard. Make sure it is in neutral when removing it. Watch carefully in the gap as it opens, and use a lift that is not going to ""spring"" upwards when it comes loose.
Jim

I agree, you need to remove the top cover so you can gain access to the differential and see what is going on .

I would pull the brake shafts(bull pinions) first to get look at the differential. Best and easiest way to get differential out is to split tractor. Lot more work to get all the hydraulics and electrical components off than it is to hang them on a sling of sort and back the rear end out from under.

Usual differential failures are when plowing with one wheel in furrow and excessive slippage on either one of the wheels. Most often happened when a customer hung a dual on land side to hold that side down.

Leave it alone & take it tractor pulling!! (lol)

I would pull the PTO out and look in the opening, hoping to see if there is a problem in there.
SDE

Tom if it was working okay when parked the only thing I can think of is rust. Hard to believe rust would stick the differential that bad. But you live in a climate that can do it sometimes. Even if in a open shed at certain times the weather is right for metal sweating. Water will drip from the housings. That said usually if rusted that bad the shift forks will also rust and stick and the sliding gears won't want to move on the main shaft.
Would try Pete's suggestion first. Take the brakes off then the plate inside shoe contacts. Then pull the bull pinion gears out. They go in the differential. Can't see the backs of the differential pinions or where they turn on the cross. But shine a light in the splines and look for metal shavings or rust. If it looks rusty you mite try leaving one bull pinion out and drive the tractor around. Maybe the vibration and oil slinging up on the differential would break things lose. Try to find a plastic lid or something to stick the removed pinion hole if doing. be carful with just the one brake. Also the tractor will stop moving if it breaks loose and gears won't hold the tractor.
If its not rusted the other posters already said what's needed.

Well it looks like a severe case of rust. pulled the left side and found a pile of rust. I'm going to vacumn it out and do the same on the other side. If you look real hard you can see it's a pile of dry rust, obviously hasn't gotten any lubrication for a long time. I know the smart, correct thing to do is take everything apart and replace and repair. But I'm leaning towards soaking it down to see if it breaks loose. If it doesn't then I guess I'm in for another major tear down.

Yes it is a serious case of rust. I will also add that rust is composed of iron oxide. in powdered form it is a very good abrasive. Letting the rust get into all the other components is a very bad idea. Flushing it out will still leave almost all the rust where it is. I am going to suggest that removing the top. and taking it apart is a very conservative method of assuring your financial relationship with the fine and dandy remaining components. The rust that will get into the oil is the rust broken up by motion when it frees up. Though it will cost the time and effort to remove the top, it will be money and time ahead to do it know. Jim