Dave,

Double/triple check the shifter cable adjustment at the transmission. I like to set the trans in 2nd gear and be sure the cable connector slides in and out of the trans shift lever smoothly. Check it again in 1st and 3rd. Takes two poeple, one shifting, one checking, otherwise you will wear yourself out getting up and down! LOL

Check the shifter cable routing. It could be getting in a bind when the engine is torqued up at full load.

What shifter are you using?

Scott