Before you go waste money on testing injectors, start with this.
-Not sure what oil you run, but in the winter 5w-40 will help with cold starts.
-Clean battery terminals, and fully charge batteries. Low voltage will cause your motor to not only crank faster, but not run at it's best.
-Check the Glow Plug Relay (they are notorious for burning up)
Winter Operation, Hard Starting, Hissing Sound Etc...
Stancor Relay Mod!!
-If the GPR is bad, replace with a Stancor relay (around $50 shipped)
-Glow plugs can be replaced for around $80 (go with motorcraft)
-Try plugging your truck in 3hrs before cranking
-A properly working GP/starting system on these trucks will be able to crank at -20 degrees F w/o being plugged in.
Let me know if you need any more help.
Oh yea, don't cycle the GP's...it does nothing but wear them out faster. Don't begin to crank when the Wait To Start light goes out, the GP's will remain hot up to 2mins depending on surround temps. Give your Gp's 45+ plus before cranking to see if it helps.