Why is my 1994 Jeep Wrangler 4 cyl having trouble going getting above 50 mph?

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My 1994 Jeep Wrangler 4 cyl std hasn't been going over 50 mph. So the highway is a nightmare need help fixing the problem. I'm going like 50 mph on the 5th gear which is the last gear on the highway and the engine sounds like its revving asking for the for the next gear but there isn't one. My grandfather sound it's mostly the transmission which I hope it isn't and friend who had a similar problem sound that it could be the catalyst converters. On a side note the engine is pretty loud.