Unfortunately this is a common problem. You'll need to remove the exhaust
manifold. When you do, replace all the studs on that bank with stainless
steel studs per Ford TSB 05-5-4:

Exhaust System - Exhaust Manifold Studs Broken

TSB 05-5-4

03/21/05

USE OF STAINLESS STEEL EXHAUST STUDS IN
SERVICE FOR 5.4L 2V ENGINE

FORD:
1997-2004 Expedition, F-150
1999-2004 F-Super Duty
1999-2005 E-Series

LINCOLN:
1998-1999 Navigator

ISSUE
Some vehicles equipped with the 5.4L 2-valve (non supercharged) engine, may
exhibit broken exhaust manifold studs.

ACTION
On the affected bank only, replace the exhaust manifold, manifold gasket(s),
and install all new stainless steel exhaust manifold studs and nuts. Torque
the studs in the cylinder head to 71-115 lb-in (8-13 N.m). Torque nuts on
studs to 204-239 lb-in (23-27 N.m). Refer to Workshop Manual Section 313-01A
for exhaust manifold removal and installation.

Part Number        Part Name
W703902-S403  Stainless Steel Stud
W701706-S2      Stainless Steel Nut