Sal,
Yes you can replace the rear main seal and oil pan gasket without removing the engine from the truck. Get a Sneaky-Pete tool and remove the rear main seal cap. The bottom seal is in the rear main cap. Don't lose the shims!! Remove the upper rear main seal with the Sneaky-Pete tool. You can also install the rear main seal with the Sneaky-Pete tool. Oil the rear main seal well and take your time with the install. Be careful not to twist the seal and don't score the seal seat. Re-torque the rear main cap to the shop manual specifications. The oil pan should drop down with a little persuasion. Install the oil pan gasket with yellow trim adhesive to the pan. Let it dry. Make sure the engine surfaces are REALLY clean before you reinstall the pan with the new gasket. Don't over torque the pan bolts. A little petroleum jelly on the oil pan gasket going to the engine will help seal the cork gasket and make it easy to position the oil pan.
Hope this helps.
Tim
