97 silverado tranny issues?

i have a 97 silverado half ton and its started to do some pretty funny things. after i drive for about 10-15 miles the check engine (http://www.gmtruckclub.com/forum/showthread.php?t=14302#) light will come on and the truck will start to drop hard into all the gears. but if i shut it off and let it sit for a little while the engine light goes off and its fine for another 10-15 miles. Another thing is that when im going down the road the whole truck will start to shake and stutter like the tranny is slipping. it shakes so bad the it spills what ever is in the cup holder but if i let off the gas and get on it again it goes away. i dont even know where to start. i have heard its everything from the drive shaft to the drum brakes (http://www.gmtruckclub.com/forum/showthread.php?t=14302#), the u joints, the tranny just needs to be rebuilt, maybe the transfer case is shot, even that the battery isn't putting out enough voltage. any help would be greatly appreciated