One thing you have not mentioned is what speeds you are getting.
If you are sitting at those signal levels and are getting good speeds then don't bother... if you can get better speeds when driving towards the sprint tower, then an external antenna might help you out.
I've had good luck with parabolic grids (I pick up the ones that go for 39.99 on sale designed for wifi and mount them 90 degrees out of phase)... one thing to remember when you go with antennas like this is you are connecting to a single tower.
If you are in an area with multiple towers you might loose the advantages you gain with multiple towers... for example your mc760 looks at the towers around it and it connects to a tower that it thinks will be best... when you have an antenna aimed at a single tower, this can't happen. Note that this isn't perfect.. sometimes the presence of multiple towers causes issues that a aimed antenna can fix as well.
You need to research Jim's site for lots of good details.
The menu of topics is hard to spot, its on the right side and down a bit on the page... lots of good topics however.
The article about parabolics is here:
»evdotips.blogspot.com/20 ··· nna.html
Good luck to you!