The new "Allison" 10L1000 in the 2020 GM's can lock in 1st gear. Lol. But yes, the 68rfe and aisin cannot lock in 2nd gear and the exhaust brake will not work without the TC being locked.So you have an Auto?
No auto has a 1st gear TC lock up. So no exhaust brake in 1st.
5-6 hunting. Are you using tow haul? Lock out a gear if not.