I've rebuild several. They are actually incredibly easy. Only one continued to leak, but I'm convinced I was sold the wrong kit with a different output seal. You just need to make sure you inspect the sealing surfaces and the sector shaft gears and worm gear for pitting. If these are present I'd get a new box.
Make sure you replace the sector shaft bearing also, this should tighten the box up considerably if the gears inside are in good shape. Google "saginaw steering box rebuild" for writeups.
You should also be able to get cheaper than $40 on Rockauto.
98 TJ 5.9 Stretched and big axles
01 F250 Powerstroke 7.3/ZF6