yes there is. its right next to the mow height dail. you can see the lever clearly in the bx70 series brochure online. i see it fails to mention it has a diff lock anywhere though. I love using my bx diff lock when needed
My wife thought she was stuck crossing a ditch at an angle. Wanted me to push... no way! I put her foot on that little pedal, and told her to back up, yer not stuck. Her eyes got real big... but wasn't stuck anymore!
If one wheel is spinning on the BXs while the opposite wheel is stopped the pin and the housing it resides in has been known to break and the pieces have also been known to jam between the differential gear set and the outer housing, usually knocking a hole in it or cracking the housing with large repair costs.
If you use the differential lock, bring all motion to a stop, press lock and while holding lock down operate hydro pedal and rotate the wheel slowly until you feel the locking lever to drop slightly and then continue with it depressed until you are unstuck.