transmission won't go past third gear on 1991 GMC Suburban 2500

transmission shifts through first, second & into third then stays in third. i am wondering if this could be related to towing. i have owned this vehicle for 170,000 out of it's 230,000 miles & only just recently towed a 2500 lb trailer of firewood. now suddenly i have a blown head gasket & transmission that won't go past third.

Asked by for the 1991 GMC Suburban 2500
get a new one and make sure its the high performence