Thinking of buying '05 Grand Cherokee with 79,000 miles, how long will it go before trade in and is there any issues for that model yr?

Don't expect that Jeep to be as reliable as your old Subarus- I would be afraid of engine failure in that model, especially if it has not been maintained properly by the previous owners-

It's hard to say how long it will last. Jeep vehicles usually are below average in reliability. Repair costs will be more frequent. Used vehicles are always a gamble.

Thanks for your answers on the grand Cherokee - any thoughts on 05 Lexus RX 330?

Lexus would be much better choice, but consider a RX-350. They are newer and a better vehicle.