HelloCustomer my name is Mike.
You did not include the year of the truck but the 4.3 is pretty much the same on most years. It sounds like you have a lower intake gasket leaking, this is a very common spot for them to leak on this engine. Below is a repair proceedure. Please ask if you have further questions.
Intake Manifold Replacement (Lower) Removal Procedure
Leave the accelerator and cruise control cables and the wiring harness on the bracket.
Important: Do not discharge the refrigerant.
Notice: Proper lower intake manifold fastener tightening sequence and torque is critical. Always follow the tightening sequence, and torque the intake manifold bolts using the 3 step method. Failing to do so may distort the crankshaft bearing bore alignment and cause damage to the crankshaft bearings.
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Tighten Tighten the power steering pump mounting bracket bolts and the power steering pump rear bracket nuts to 41 N·m (30 lb ft).
Tighten Tighten the AIR reactor pipe bracket nut to 41 N·m (30 lb ft).
Important: In order to install the distributor for the correct engine timing, position the engine to number one cylinder top dead center.
Tighten Tighten the accelerator cable and cruise control bracket nuts to 9 N·m (80 lb in).
Tighten Tighten the accelerator cable bracket studs and nuts to 12 N·m (106 lb in).
For someone that has done one before I would call it about a 6. If you have never done it it's more like an 8-9. It's not terribly difficult but labor intensive. There is risk of getting the engine out of time if you do not mark the distributor directly and risk of dammaging the injectors if you are not careful takeing them out.
I don't have access to parts pricing at home so the best I can do for you on a price is an educated guess. I would guess around $150 in parts and 5 to 6 hours in labor. Everyone's labor rate is different, the shop I work at charges $93. So my labor rate plus parts would be around $700.
There are some things I would say definately go to the dealer for, this isn't one of them. Any competent shop should be able to complete this job just as well as the dealer and will probably come in at a much lower price. Please ask if you have further questions.