[94 Jimmy]

Check out the Haynes Manual for your Van, it costs about 20 bucks and is worth every penny.
I just did the head gasket on my 1998 3.4, it's not a joy, but it's possible if you take your time and lots of pictures. On the 98 you have to rock the engine forward, if that's what you have to do on yours also, remember that if the rope holding the engine forward lets go and your arms are back there, it will porbably break them, double up on the tie down straps used to rock the engine forward.
Regarding the shop check, I found a crack between the intake and exhaust ports in #6 cylinder. I had it checked out and all of the intake valves were leaking and the valve seals were leaking oil. Both heads cost about $160 and about $180 for a gasket set (don't use FelPro, my last head gasket lasted only 50KMi)
Good luck, get that manual
See ya
94