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The rear main oil seal in your vehicle is a 1 piece seal & the transmission must be removed to install it.Here is the information you asked for.
4.3L, 5.0L, 5.7L and 1991-98 7.4L Engines
See Figures 10, 11, 12
Special tool J-35621 (or J-38841), or its equivalent seal installer, will be necessary for this job.
Fig. Fig. 10: Rear main seal and retainer-4.3L, 5.0L and 5.7L engines
Fig. Fig. 11: Oil seal removal notches
Fig. Fig. 12: Oil seal installation tool
6.5L diesel engines
See Figure 13
Fig. Fig. 13: Using tool J-39084 or equivalent, drive the seal in until tool bottoms out against the block and the crankshaft rear main bearing cap-6.5L engines
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See Figures 1 and 2
It would be best to drain the transmission before starting.
Installation is the reverse of removal but please note the following important steps.
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.
Fig. Fig. 1: Automatic transmission and related components-THM400, 4L60E and 4L80E
Fig. Fig. 2: Automatic transmission and related components-THM700R4