its a tough call with the old truck comes the not if but when something fails and needs repair. with a new truck comes the BIG $$$$ cost of buying it, not just limited to monthly payments but insurance, tax and reg fees.
Yes i like having a new truck every 5 or 6 years, i try to pay it off before trading in. i personally have not had emmission related issues yet with the new diesels and my prior 2011 diesel had over 120k miles.
This is the advice i gave my father regarding his 2007 GMC drw, he wanted to buy a new one last year since his is getting older and becoming problematic. I told him if it were mine(his has 37k miles) i would take what his monthly payment would be and buy parts, replace the worn items and stay with that truck for a while longer.