Choke is not the correct word or concept at all. "choke" is a butterfly valve in the intake a carb'ed gas engine that closes to pull in more fuel during a cold start. Try again.
Anyway, what you're describing sounds very much like your Exhaust Back Pressure Valve, or EBPV. It's a hydraulically actuated butterfly valve in your exhuast that your engine closes to raise backpressure to help the engine warm up in cold weather. Yours sounds like it may be sticking, as mine will usually open right up when I press the accelerator, and close back when I'm at even throttle, coasting, or idling. The actuator rod gets all gunked up and even leaks sometimes. A lot of people just delete the whole thing if they've got the turbo out.
If you do a title search on EBPV, you'll get enough hits to keep you busy for days.
edit: BTW, we dont have a blowoff valve. They're needed for turbo'd gas engines to allow boost to escape when the throttle plate closes...and we ain't got no stinkin throttle plates.
2002 Arizona Beige F250 Ext Cab Short Bed. 7.3L 6spd 4x4 140k
2" leveling kit, 285/75 Mastercraft AT2's,Napa BFF, DiPricol Pryo/Boost, DP F5 NS/stock/hi/40/80/120/140agg-lope, Toolbox, Audio goodies, IH shiftknob
muffler delete, CCV reroute, AIH/Foil delete,