99 Yukon Replaced the Fuel Pressure Regulator and now.....

Ok I had my truck into the shop last week and They replaced the FPR and wanted to do an injector cleaning for $100. I declined the injector cleaning hoping to run some Injector cleaner through a tank of gas.
When I got the truck back it started 100% better but ran way way worse. It is now misfiring so much that I have conciderably noticable loss in power. The truck doesn't seem to be shifting the same, probably due to the loss of power. Anyway is it possible that a working FPR would cause the truck to run conciderably worse. Before the FPR was replaced I had extended crank and a rough idle but plenty of power and it was responsive. Now it is rough idle no power and un-responsive. Any advice? Could they have put the distributer on a little off or something. I heard here that this could cause a problem if you weren't careful on these engines.
TIA Corey
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