Does anyone know what a line pressure solenoid valve circuit does and how much a replacement would cost?

Are you getting an code for that? It is part of a group of 4 solenoids located within the valve body. You could get the group for about $100. You will also need a tranny gasket and you might as well replace your tranny fluid. Not too bad of a job. Also a good idea to have about 4 cans of brake fluid to clean the tranny pan.

thanks for the info. are there an symptoms (some hard shifting, perhaps) that this faulty circuit pack exhibits besides the self-diagnostic test? Any idea how long a replacement would take, what is involved, or where in the tranny the circuit pack is located?

If you already ran the diag and did not get any codes you might still want to consider changing them out. If you did not run the diag look at the stickies for instructions. Pretty simple.

You might also want to consider getting a grounding kit from Matt Blehm. That helped my shifting.

You could still put in a new solenoid pack. Go here for the pack. First one listed.
http://www.bulkpart.com/Merchant2/me...Code=RL4FO2Asw

Good luck and God bless.