Well you didnt say if it was a V6 or a 4cyl, but the V6s are recommened to be changed every 105k and i would recommend changing the 4cyl somewhere between 100 and 150k to be on the safe side. Remember 4cyl is a timing chain and the V6 is a belt. As far as the 4wd goes, jack up one front tire and one rear tire at the same time and engage the 4wd, put it in gear and ease off the clutch. Be very careful when doing this as not to fall off the jack. The tires in the air should turn signifying the four wheel is working. NOTE* this will only work if the truck has open differentials front and rear. The hubs are extremely easy to install if you are going with manual locking hubs, it took me about 30-45min. to install my warn premium hubs. Sounds like a fair price to me. Check for rust spots on the frame and especially in the bed. Check the exhaust manifold for cracks....ummmm thats all i can think of right now besides checking the oil and pull a few of the spark plugs to look for tell tell signs of problems.