You can get a new rebuilt box (A1 Cardone) from Advance for $132.99, with a core charge of about $30. So, if you don't have a core, the full price would amount to $162.99, which is not much more than it would cost to buy that one for $100 and replace the seals.
It has a lifetime warranty, which is great since the seals in these boxes are bad to leak after a few years.
I know someone who used to work for Cardone, and their quality control is superb!!
I put one of these on my truck, and I'm well pleased!!!
That may be a better route, if you've got the correct pitman arm.