I just recently replaced intake manifold gaskets on my 2002 f150 4.6. After installation I started the truck to find it running very rough and back firing into the intake manifold. Any suggestions as truck ran fine before.

 

I'm not experienced on that model F-150, but if you don't get any help; I
would do a search on Ford Truck enthusiasts.com website.
The forum has a lot of Ford Truck technicians and your problem may already be on
a thread there.
Did you already double check all electrical connections to injectors and solenoids and vacuum hose connections? I would imagine you did, but go back carefully and look for things that might be hidden, like a cracked vacuum hose or something like that.
Sounds like its not running in the correct firing order.

 

There is also a good possibility 2 coil plugs may have been switched.
A wiring diagram will provide the color of the correct wires to each # cyl coil unless you made notes of each cyls coil plug.