Jeep Repair: 1998 Grand Cherokee, crankshaft sensor, radio hum

Question
My 16 yr old son just bought a 98 6 cyl Limited with 131k.  The a/c slowly leaks out.  Do you have a methodology when trying to trouble shoot leaks?  And...if I get a can of R134 and the compressor is off because of a low level condition, do I have to bypass the low pressure switch before trying to add new freon?

The rear window will lock and unlock and open but the rear door handle will not open the back hatch.  I crawled in the back and took the 4 screws out but it did not seem to loosen the back interior hatch panel enough to get it off.  Should I do that again and try more force?  I'd hate to break it.

What oil would you recommend in the straight 6 engine?

We need this car to last.  The previous owner just had a crankshaft sensor, plugs, wires, cap and rotor put on and claims the transmission was done last year by AAMCO.  This is the quad trac full time 4x4.  Is there anything you would do next to the drivetrain and differential?

The car will not have to pass emission tests after next year.  Would you scour out the catalytic converter to reduce back pressure?  A friend told me if I "rodded" out the converter I could get 2 or 3 more miles per gallon.  Is this bogus?

I've checked the belt,replaced the air filter, topped off the power steering, brake, and coolant fluids...anything else you would do under the hood?

There is significant radio hum going on through the stereo speakers.  Is there a condensor like the old days (with points) that I need to look for or do you think a ground wire is not making a good connection.  If there is a condensor where is it?

Thank you for helping.  My son bought this Jeep with money he has earned and saved since he was 12.  The exterior and interior are in good shape and I want to make sure I do all I can to keep it running.  It will have to last him the rest of high school and college.

Thanks again...Terry

Answer
evaps leak for the ac system
rear door lock actuators  go bad back there.
what ever the owners manual says   10/30  i think.
Do diff service  every 20k all of them
if u clean out the cat  u will be certain not to pass the emissions.
The radio would not have good ground.
I live in the Atlanta area if u would like I can look at this for you.