Transmission seal is leaking. It appears

To replace the front seal, it's a bit involved and you'll need the proper equipment and tools: Floor jack, transmission floor jack, jack stands to get the truck elevated enough to pull the tranny.
IF it's an automatic, you'll have to take the bellhousing loose from the motor, driveshaft(s) out, remove tranny and torque converter, all lines, linkages, drain oil.
If you don't have much mechanical experience, you may want to have someone else tackle it for you. It's doable, but it's a bit of work.
If you're going to attempt the repair, I would recommend purchasing a repair manual such as a Haynes, or possibly a CD service manual. The step by step instrucitons would get you through the job, plus you'd have access to oil and torque specs to do the job correctly.
ps: if this is the REAR seal, it's a simple job of dropping the driveshaft and replacing the seal.
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