I recently aquired a 1999 GMC Envoy. Ive heard they are fairly similar to a Jimmy or Blazer so I decided to ask this question on here in hopes that someone might be able to give some advice. We took the truck out on a three hour drive the day after we got it, and about halfway into that drive it started shifting very poorly. The truck would basically slam from first into second gear. Today when we got home, i looked up in the owners manual what the second button on the shifter was for. Ive never owned any suv besides a nissian xterra and those do not have a second button on the shifter. Anyways, that button is for a tow/haul mode. Im sure experienced owners of this type of vehicle would know how that tow/haul mode would effect the shifting of the vehicle if it was engaged without pulling a load. No light comes on the dashboard when you hit that button, but whos to say thats working properly. Anyways, could that tow/haul mode cause inproper shifting if engaged without a load on that truck and if not, what could be causing that issues besides a totally bad transmission?

Thanks in advance.