the 91 is a bit more powerful , I have read there is about a 2 hp difference in the years you have , also like MAC said , the 4x4 is 4x4 all the time so that puts a drain on it , get the 4x2 kick out , you will love it , then you late model might be a bit more worn out , do a compression test , my wife's 90 engine in a 96 frame was a totally different ride compared to my 95 , after a fresh jug , early mod cam and a Rick's CDI I'd say I caught up with her , but I stretched out the timing chain in 35 hours from not changing the cam sprocket ( I hope ) , that is a work in progress , parts suppose to get to me Monday , hope to be running it next weekend