I'm not real familiar with the '99's, but here are the answers I know:
I've been told that if I find a 99 Explorer with a 5.0, it will be a perfect doner car with all of the excessories provided. Is there another ford similar with the 5.0? (ie. F-150...)
The 5.0 was used in many applications, but I don't know how well those applications will swap into an RBV. Unless you get something concrete from someone, I'd stay with an RBV donor. The 5.0 was used in Explorers from '96 to '01.
Can I use my PCU or do I have to use the PCU out of the 5.0 doner car?
You'll need the PCU from the donor vehicle.
More than likely, I will be converting from an Auto tranny to a 5 speed; but in case I don't do that conversion, will my 4.0 liter, auto tranny bolt up to the 5.0?
No. Also note that, in Explorers, the 5.0 versions came as AWD versions (maybe some 2WD versions). You don't say if your Ranger is 2WD or 4WD. If 4WD and you want to keep the locking transfer case with a low range, you'll need to figure out what t-case will bolt up to a 5.0 transmission and still fit within your Ranger's frame rails.