1990 Jeep Laredo Transmission cooler lines to transmission

My 1990 Jeeps transmission cooler lines have rusted through and are leaking fluid. The local mechanic gave me a quote of $700.00 to replace them. Could I replace them my self and would it be difficult? [ I have the time and tools] Can I use something other than steel lines for a replacement? This vehicles engine runs like a top and I feel I could get another 100,000 miles out of it. I seen some these 4.0 liter inline 6's with over 350,000 miles.

Thanks