Get a scan gauge and check your boost pressure. Under WOT you should be getting north of 20, closer to 30 actually (can't remember exactly) . We had a boost sensor go on ours and it limited boost to under 20. It was a dog. It was not expensive to find or to replace....according to my mechanic.
Also check your exhaust manifold for cracks. Easy to see. Just lift the bed and look closely. We had one of those as well which caused a lower boost and subsequent lack of power. Exhaust manifold re and re was under 900 bucks Canadian including parts.
Take a look and see what you see. The crack will be obvious and its common with the ISC.
