Just replaced the PS pump on my 87 4.0. The PS rebuilders recommend (or insist) that an in-line filter be placed in the return line to the pump or it voids the warranty.
I got my pump from Rockauto and went with one remanufactured by Delco-Remy (at double the cost) because it was a Genersl Motors pump originally. Reading the description of the services it was more thorough than the others. My thought was they should know what they're doing because they made them originally. There are too many rebuilders out there taking shortcuts and putting out bad parts.
As a side nore, a sticker said it was remanned in Mexico and that's ok with me as long as it is quality work. So far, knock on wood, it's still good after 100 miles.