So far I really like my 87.... but I have some concerns

[chevymotocross]

I really like my 87 silverado 2wd swb. I've had it for about a month now, and it's a big improvement over the 78 f150 I had. The truck seems to run really good and gets 15 mpg consistently, which is outstanding compared to the 9mpg I was getting with the ford. I notice whenever on the highway, after about 20 min of steady driving, the service engine light comes on. It never comes on during city driving, and all my fluids are fine, no overheating or abnormal running conditions that would hint me to pull over and get a tow truck. Many have told me it could be a faulty EGR.... but is there any other way to tell? Also, overdrive is great. I can cruise at low rpm and the truck is good on fuel. Problem is, when the truck is empty, on slight uphills, the thing loses speed quickly, downshifts, does it alot. When just towing my dirtbike on an open 5x8 trailer, forget it, the thing does not want to stay in od. It's only a 305, I know it's not a powerhorse, but it really seems to run out of steam on the highway on uphills.... I'm guessing it has a very high gear ratio like a 3.08, that could be a reason but is there anything I can do to give it some more pep on the highway? It has dual exhaust manifolds back with brand new cats.