I have a 91 with the 4.0, automatic. I put on a TJ Front pipe with a highflow cat and a Flowmaster 40 series, currently no tail pipe. It sounds great, gives it a deep throaty sound without being to obnoxious. Bought everything from Summit, the Cat includes the front pipe and it runs around in front of the oil pan instead of hanging under the tranny gaining great ground clearance. Here are links to what I bought:
Cat and Pipe: http://www.summitracing.com/parts/MPE-23227/
I also bought a new donut gasket at the local parts store and I tried to put a stock tailpipe on it but it was to short, probably could have bought an extension pipe and stuffed it in there but I figured I would just go have a one piece tailpipe made for it, that was over a year ago and I still have not gotten around to it. I also had to get a plug for the extra O2 sensor hole in the cat, front pipe had the O2 hole for the single sensor the YJ has.