In the last two weeks it's gotten slower and slower cranking to start. If I try to start it four hours after shutting down it starts just fine, cranks full speed, etc. If I try to start it after sitting for 8 hours or overnight it cranks slow, and its getting worse and worse by the day.

Would batterys fail/deteriorate this quickly?

 

I recently changed both my battieries due to one failing. I had no starting issues until I had let my truck sit for a few days while out of town. Come Monday morning when I left for work, it would not start. Had to charge the batteries for 1/2 hr and use the charger's "boost" to get'r going. I tested the batteries, and charging system, (alternator-OK) after charging them for a few hours that night and found a 4 bad cells in the driver's side battery. The passenger side had been weeping acid around the top, so both were in need of replacement. Now, all is well again.
In short, "YES" you could have bad batteries. I would have them and the alternator tested.

 

Probly did this already but make sure all connections are clean & tight. How old are batts?

 

They're clean and tight.
Not sure how old they are...bought the truck in december with the batts in it.

 

There should be a date code stamped (or heat-branded) into them somewhere so you CAN tell their age. After about three years, or a few accidental discharges, the batteries are a real suspect.

It sounds like the reserve capacity has diminished to the EOL (End of Life) point. Eventually the sulphation process takes its toll on all lead-acid batteries. New ones will most likely solve your problem,

However, if the alternator has over about 120,000 miles on it, it might-well be "gone through" by a competent local rebuilding service when you've got a couple of down days.

That's NOT the same as swapping it out at the parts house!

A healthy vehicle electrical system isn't accidental.

Pop

 

You can check each cell with a hydrometer after you fully charge the batteries.. Any weak cells, replace both batteries... you can buy a hydrometer from napa or most auto parts stores for under $20..

 

Disconnect the batteries and test their voltage. If they aren't the exact same, that is your problem. The lower one has bad cells and is pulling a charge off the better one to try and charge itself.

 

Replace in pairs, right? Or can I get away with one?

 

Also replace both batteries..

 

Have you checked the voltage at the batteries at an idle ? Your alternator should be putting about 14.3 volts out.. Also, you might put your hand on the alternator after the truck sits over nite .. if it is warm, your alternator is going bad..

 

You can replace one now but that will probably cause you to replace them again within a year. Always do both (I learned from experience).