Help w/ VSS & Oil Leak

Hello my best gen brethren! First post here...

I am finally in a position to start rebuilding my '99 4x4/2.7 regular cab after picking up a commuter car so I can garage my Taco and tackle any task that takes longer than a weekend.

My truck is in pretty decent shape after 235k miles, the engine runs strong and I can't find any rust on or in the frame since I'm from the PNW and I believe the truck has been here for life.

First things on the agenda are definitely ball-joints, I'm pricing those out now but I also need some help with a couple other problems. A couple months ago driving home from work I looked down and noticed that the speedo and tach weren't working at all and then 20 miles later I got a CEL with a PO500 code which I believe was for a faulty Vehicle Speed Sensor. Is this the issue? Just the sensor mounted on the side of the transmission? If so, I can't see how that would affect one of the two gauges but not the other. Could it be a different issue? The OEM part is crazy spendy so I don't want to throw sensors at this issue only to find out I missed a blown fuse somewhere.

Other problem is a pretty significant oil leak that appears to be coming from the oil filter housing. Is that a common place for a leak to occur? I just did the valve cover gasket in May and don't see any oil coming from there and the source looks to be coming from the area under the exhaust manifold.

Also that leads to a third issue I found when pulling things out to find this oil leak. I have two large cracks in the Exhaust manifold. It seems that the consensus here is that headers won't do squat so should I stick with an OEM manifold?

After these are fixed, and probably after hunting season this year, I'm going to start throwing money at new suspension as it's all shot to hell and I'm sure I will have many more questions for y'all.

Thank you!