I have the 4.0 L V-6 and am having the same problems you describe.
- never overheats
- bubbles only after it's shut off
- rarely used coolant
- replaced the cap - didn't help
- pressure tested the radiator while it was running, and the pressure was climbing above 16 psi even when cold (the cap is rated @ 16 psi, so something is overpressurizing the cooling system.)
- the smell of coolant after it's been running awhile, yet no leaks
- no white smoke out the pipe
All of these point to a cracked cylinder head, or a blown head gasket. I did not perform a leak-down test or compression test, but I'm 99% sure it's the head or gasket.
I'm in the middle of tearing down the top half of the motor. I have the intake off and will pull the heads in the next day or two. I will post anything I find with mine.
Good luck !