Gmchevrolettruck

This truck only has 1,2,3, & drive, not a tow mode like my newer one. Have had to run it in 3rd because of jumping back & forth, burn smell this morning. Thank you guys for your help

Hello, I have a 1999 Chevy 2500 (looks like old style) with a 350 engine, 4 speed automatic, 8 lug wheels. Not sure of the gear ratio. Pulling a 3000 lb trailer 1000 miles mainly over flat land. Was wondering if to leave it in drive or put it in 3rd for pulling. Thanks, Jay