Hopefully someone else will have a suggestion on what to try.  Mine did
the same thing.  Dealer replaced it twice before it finally quit locking
at wrong times.

 In addition to normal restraining, the inertia reel-type belt is designed
to ratchet back (lock) when the belt is fully extended and released. The
design allows a child safety seat to be placed in any passenger seat, front
or rear, and secured without rocking. Try it on all the passenger belts.

 As long as you don't fully extend the belt before buckling up, the belt
should give you freedom of movement.

 This locking up is a common complaint at dealerships and replacing the
belt doesn't always resolve the concern. If you fully extend the belt before
you buckle, it will always lock up. If the passenger is large in girth and
the seat is moved forward, this makes this condition worse.

 All that said, these belts do seem to get more sensitive to locking up as
the vehicle ages possibly from dirt or grime or that can of Coke that fell
off the seat when you turned left. I was a passenger in my daughter's '01
Rodeo yesterday and could not get the belt to loosen up but with 90,000
miles and a couple of years of Mickey Ds and Taco Bells, well you get the
picture.

 You could probably pull the reel out and spray the mechanism with brake
cleaner and it'll probably work fine. Good luck!

 Joe D

 >I have a 1999 rodeo. The passenger side seatbelt locks up making it
 > pretty uncomfortable for passengers. You had better not sit back or you
 > are locked there until you undo the belt. Is there a simple way to fix
 > it so it won't lock until it's supposed to?
 >
 > Eric