Ok well the transmission and tranfercase must be removed to do this procedure. Here is what will need to be done. If you let me know if this is an automatic or manual, I can get you more info on the transmission removal. Feel free to contact me with any questions! Good luck!
Remove the transmission.
Fig. 10: Rear main seal and retainer-4.3L, 5.0L and 5.7L engines
Fig. 11: Oil seal removal notches
Fig. 12: Oil seal installation tool
With manual transmission, remove the clutch.
Remove the flywheel or flexplate.
Remove the oil pan.
Unfasten the oil seal retainer screws and nuts.
Remove the oil seal retainer and gasket.
Insert a small prying tool in the notches provided in the seal retainer and pry out the old seal. Be VERY CAREFUL to avoid nicking or scratching the sealing surfaces of the crankshaft.
Install a new gasket on the block and install the oil seal retainer. Tighten the screws and nuts to 11 ft. Lbs. (15 Nm).
Install the oil pan.
Coat the inner and outer diameters of the new seal with clean engine oil.
Using seal tool J-35621, or equivalent, position the seal on the tool.
Thread the seal tool attaching screws into the holes in the crankshaft end and tighten them securely with a screwdriver.
Turn the installer handle until it bottoms.
Remove the tool.
Install the flywheel/flexplate, clutch (if applicable), and the transmission.
Ok here is how to remove the transfer case.
See Figures 1, 2 and 3
Raise and support the truck on jackstands.
Remove the skid plate if necessary to gain access to the front seal.
Matchmark and remove the driveshaft.
Remove the nut, washer and yoke.
Some models don't use a washer at the rear yoke nut.
Some models employ a shield around the seal. Remove it.
Fig. 1: Installing the output shaft seal
Fig. 2: Transfer case and components-NP241
Fig. 3: Transfer case and components-BW 4401 and BW 4470
Using a hammer and a punch, carefully distort the seal so that you can pull it out with a pliers. Be careful to avoid damage to the seal bore!
Coat the seal lips with clean oil fluid. Coat the outer edge of the seal with gasket sealer.
Position the seal in the bore and drive it into place by carefully tapping around it with a hammer. If you have access to one, a seal installer will make the job a little easier.
Install the shield, if used.
Coat the outer surface of the yoke neck with clean oil and slide it into place.
Install the nut and washer. Tighten the nut to:
NP205: 150 ft. lbs. (203 Nm)
NP208: 120 ft. lbs. (163 Nm)
NP241: 110 ft. lbs. (149 Nm)
NVG243: 110 ft. lbs. (149 Nm)
BW 1370, BW 4401, BW 4470 front: 165 ft. lbs. (225 Nm)
BW 1370, BW 4401, BW 4470 rear: 125 ft. lbs. (170 Nm)
Install the driveshaft.
Install the skid plate.
Here is how to remove the transmission.
See Figures 1 and 2
It would be best to drain the transmission before starting.
Disconnect the battery ground cable.
Remove the air cleaner.
If equipped, disconnect the detent or TV cable at the throttle lever.
On 4WD models, remove the transfer case shift lever knob and boot.
Disconnect the shift linkage from the transmission.
If it is necessary to disconnect the fuel line, relieve the fuel system pressure. Disconnect the fuel lines.
Remove any skid plates that would interfere with transmission removal.
Drain the transmission fluid.
Remove the driveshaft(s), after matchmarking its/their flanges.
Detach the speedometer cable or Vehicle Speed sensor (VSS) connector, downshift cable, vacuum modulator line, shift linkage, throttle linkage, electrical wiring, the fluid cooler lines, or any other component that would interfere with transmission removal.
Remove the dipstick tube.
If necessary, remove the starter motor.
If necessary, disconnect the support bracket at the catalytic converter.
Support the transmission on a transmission jack and unbolt the transmission rear mount or the transfer case adapter from the crossmember.
Remove the crossmember.
Remove any transmission support braces. Note their exact positions for installation.
Remove the torque converter underpan, matchmark the flywheel and converter, and remove the converter bolts.
If necessary for clearance, move the exhaust system aside.
If necessary for clearance, disconnect the parking brake cable.
Remove the transfer case and the adapter.
Support the engine on a jack and lower the transmission slightly for access to the upper transmission-to-engine bolts.
Remove the transmission-to-engine bolts and pull the transmission back. Rig up a strap or keep the front of the transmission up so the converter doesn't fall out.
Installation is the reverse of removal but please note the following important steps.
Tighten the transmission-to-engine bolts to 35 ft. lbs. (47 Nm).
Make sure that the torque converter-to-flexplate matchmarks are aligned, the torque converter is flush with the flywheel and the converter turns freely by hand.
On all except 1996-98 models, tighten the converter bolts to 50 ft. lbs. (68 Nm).
On 1996-98 models, tighten the converter bolts to 46 ft. lbs. (63 Nm).
Tighten the transmission support brace bolts to 35 ft. lbs. (47 Nm).
On all except 1991-98 models, tighten the crossmember retainers to 35 ft. lbs. (47 Nm).
On 1991-98 models tighten the crossmember retainers to 56 ft. lbs. (77 Nm).
On 1988-94 4WD models, tighten the transfer case-to-transmission attaching bolts to 24 ft. lbs. (32 Nm) and the transfer case-to-frame bracket bolts to 35 ft. lbs. (47 Nm).
On 1995-98 4WD models, tighten the adapter-to-transfer case and adapter-to-transmission bolts to 33 ft. lbs. (45 Nm).
Fig. 1: Automatic transmission and related components-THM400, 4L60E and 4L80E
Fig. 2: Automatic transmission and related components-THM700R4