Hyundai drive shaft - Motor S Hyundai Cars & Trucks

This shows it as a standard hex bolt.

https://www.wholesalehyundaiparts.com/p/hyundai__Santa-Fe/Drive-Shaft-Bolt-Drive-shaft-rear-bolt/13452602/49129C5200.html

All home mechanics need a full set of Allen keys, wrenches and sockets.

Video that may help.

https://www.google.com/search?q=2006+Hyundai+Santa+Fe+3.5+replace+rear+drive+shafts

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Drive line? You mean drive SHAFT?

Videos

https://www.google.com/search?q=2003+Hyundai+Santa+Fe+what+size+wrench+to+take+off+rear+drive+shaft+nuts

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Exactly which driveshaft do you mean? The one that goes front to back, (prop shaft), or the driveshaft going to the actual wheel?
If you are going to remove any of the drive/prop shafts you will need to drain the transmission oil. This will stop it dribbling all over the floor when the shaft comes out of the transmission! REMEMBER to Refill the Transmission BEFORE you drive it!
The 'prop shaft' is the easier of the two to remove BUT they are VERY heavy so have someone help. One end is fastened to the Differential flange with 4 bolts, the other sliding on a splined shaft at the transmission end.
Driveshafts to the wheel are a lot more work. You have to strip the lower half of the suspension to get the shaft out and the 'castle nut' at the hub end can be very tightly fitted.

Do the injectors have power to them and a ground pulse command signal ?

forth gear is not a gear as such but a dog that connects the input and out put shafts to make a straight through drive
reasons for jumping out of gear include gear not properly selected __check shifter connections or cable adjustment cable loose at one end not making full engagement of the dog
loose n bell housing bolts or bolts not tension in the correct order
bearing failure on input shaft , out put shaft or spigot bearing between the shafts
if you have had no problems before today, the shifter mechanism and cable is a prime suspect for failure

You should be able to change the shaft seal just by removing the half shaft.

The drive shaft is what connects the transmission to the rear wheels

drive the car in 2nd speed gear at1000 - 1200rpm to form number 8 shape like zig zag manner,if you find taka taka taka noise,the drive shafts are weared out.

Need to separate lower control arm ball joint from spindle assy, and then pull it out of the way for shaft to clear. Need a ball-joint removal tool, like tuning fork style, or proper puller/press tool.

probably needs new or rebuilt drive shaft.