I have a Phillips BodyGroom Pro like
this for my trimming requirements.
I didn't like shaving my legs completely as I didn't like the irritation on the back of my thighs, and... well it's a bit of a hassle to keep on top of it.
I use the bodygroom everywhere

and you can choose the length you want it to go down to. I use the lowest setting on the bottom trimmer for the legs.
When not trimmed, I am fairly hairy but never had a problem with rising leg grips to a massive extent. You're always going to get a little bit of movement even with good quality silicone grips due to the nature of the activity.
Are your shorts definitely sized correctly?