Hi, there are a few things to check.
1. locate the boost control relay and unplug it. Now, if the problem was related to ECM, wastegate plumbing or wiring, it will go away and boost will be limited to ~8psi 'base boost'.
In this case, first inspect the small tubing and elbows that connect the wategate solenoid (mounted on the turbo) to the compressor and wastegate actuator. The elbows tend do heat-harden and fail. These tubes are under pressure (not vacuum) so they can also blow off the nipples. The wastegate solenoid is the next thing to check. Its sort of hard to test, probably better just to spring for $15 and buy a new one. Only suspect ECM or wiring after absolutely ruling out the stuff above.
2. if problem still exists with the relay unplugged, you probably have a stuck waste gate. See if you can move it freely by hand. If you can, it may only stick when hot. Yes, you need to do it. Removing the fenderwell gives free access, so you can get it hot and check it without getting burned. If it moves freely, then it probably just needs to be adjusted. Lengthening the actuator rod decreases boost.
Also, if you have a stainless exhaust system, it is not stock. You sure the cat converter has not been removed or gutted? This is a sure way to cause overboost.
If you don't have the manuals, etc to locate this stuff, you need to get them. Very few people know how to work on these trucks, so you should learn it yourself. Go to
www.sportmachines.com to get a CD with all the relevant manuals on it, and go to
www.syty.net and search, lots of info there.
Hope that helps.
Oh, and on the manifolds... you can get stainless headers at
www.raceprovenmotors.com. Very pricey. Most folks just live with the iron manifolds (and some run the 1/4 mi in the 10's doing so!) or get them ceramic coated.