98 Bravada 1-2 hard shift problem

I have a 98 Bravada with 111,000 miles on it. 90% of the time the tranny shifts perfect but after I've been on the freeway it shifts extremely hard from 1st to 2nd gear only. As long as I stay on the city streets it never does this, only after hwy. driving. It has had the P1870 trouble code before after a 146 mile trip. The code is not showing up now. Should I follow the recommended repair procedures from the GM service bullitins posted here?
I have only had this truck for 3 months now and it appears to have never had the pan off the tranny for service.

I have a 98 Bravada with 111,000 miles on it. 90% of the time the tranny shifts perfect but after I've been on the freeway it shifts extremely hard from 1st to 2nd gear only. As long as I stay on the city streets it never does this, only after hwy. driving. It has had the P1870 trouble code before after a 146 mile trip. The code is not showing up now. Should I follow the recommended repair procedures from the GM service bullitins posted here?
I have only had this truck for 3 months now and it appears to have never had the pan off the tranny for service.
Welcome to AF. This a common problem with GM transmissions with the aluminum VB, most of the time it is caused by a pressure loss thru the tcc valve and bore, in the VB. This can be corrected with a modification to the VB , done by most transmission shops. To verify the problem a road test and scan sould be done to look at performance info, you may need additional work in the trans.