Recently bought this truck used (good deal) and was changing the oil when I noticed quite a bit of dirt and oil build up on the bottom side. Checked the oil levels but I haven't noticed any less oil from the dipstick. I can't find any leak on the top side or the front. It looks like the wetter grime is towards the rear middle portion and below it. I looked at the valve cover on both sides but didn't really notice around it, nor did I see any smoke. I also noticed kinda of a line of wet grime running down the drive shaft. Guessing the leak is just moving with the momentum of the truck at highway speeds. If you guys have any clues, let me know so I know where to start with this thing. I really don't want to have a mechanic look at it because of the money. Truck only has 138k on it. Timing belt, water pump, and clutch were recently replaced.

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When you did the clutch, did you change the Engine Rear Main Seal? If so, did you use a Toyota seal? Look underneath the truck, at the transmission bell housing, and find the inspection port (small hole at bottom of housing where it meets the inspection metal plate at the rear of engine) and use your finger to see if there is oil coming out the hole. If so, you more than likely have a rear main leak.
Other oil leaks are usually from the valve covers leaking at the rear of the cover, and / or the rear cam seals that are plastic disks at the back of the head, just underneath the valve covers. An mechanics inspection mirror from the auto parts will enable you to visually inspect this area.