Stiffness would normally mean something in the linkage needed lubrication or adjustment. Assuming you got the shifter into reverse after a few minutes of the engine running (and the tranny cirulating and heating the fluid) it is probably fluid related. Try pulling the top fill plug on the tranny pan when hot and see if any fluid runs out. If not, it is low and will need to be topped off and probably fix the problem.

Another clueless Isuzu Dealer - but then I guess they are in the business of selling you expensive powertrain parts. Can't change ANY fluid too long and the 20K severe service fluid interval means 40K of normal driving at best - I change mine every 10-15K to be safe.