98 Burb Steering Problems

My 98 Burb is having some steering issues. As I'm turning it wants to what only I can describe is 'straighten out'. The wheels feel like keep 'wandering'. I've had the the pitman arm changed, a new steering box installed, I've even put a new EVO (steering speed sensor) on. In my limited knowledge, it seems that the only 2 things left is the power steering pump or the tie rods.
Does anyone have any tips on how I can 'test' these without having to replace both?

are you able to check to front suspension/steering??put it on stands & see if there is
any play.i'd also inspect the tires.why were the parts replaced??were we guessing
or were they worn??whats the miles

To check for play in the steering / suspension, do I move the wheel side to side and see if the steering wheel moves? It's not real clear just how to test for play.
My mechanic thought the pitman arm was causing the problems, but when he replaced it he saw real bad seals in the gear box. So we decided to replace it as well. They did all this when they put a new engine in. The old one blew a head gastket.
The chassis has 140,000 miles on. The front tires are 'cupping' real bad and wearing improperly. I don't really notice an alignment problem though.

Your mech did not suggest idler? Idler usually gives your symptoms, I would check that. Ball joints cause tires to seat on side of tire so it wears on in or outside.To check idler jack one side Pass side is better since thats the side idler is on and will show the play better. With rt tire off ground take your rt hand grab front of tire left hand on back of tire and try to move by pushing in front as you pull from back then visa versa. If there is more then half inch play idler is bad. To check ball joints change hand position to top and bottom of tire if bottom moves in and out bottom ball joint bad if top moves then top bad. You may need extra hard push for ball joint its harder to move tire. Good Luck