On my 98 grand Cherokee Laredo I had symptoms of a bad alternator. Ii swapped it out and everything seemed good with the multimeter showing 13.9 volts. I let the car run for a bit and then tried to restart it am worse than before. When hooked up to the terminals and the car off the battery reads way less than it does when out of the car. I regularly get a different reading going from battery post to post than I get from going anywhere on either terminal connection

 

Are they the original battery terminals? if they are start with new terminals before going any further.

 

I just used those cheap bolt-on clamps to replace mine, but they've lasted 50K miles already. The BEST ones I've seen were solid brass marine clamps that had a screw post and wingnut, but those are about $35 each! And need a ring-tongue terminal on the cables.