My father use to say if it is not broken don't fix it and save for when it does break. Start putting some money away in a savings account and keep driving with your current engine, I am going tell You when I was looking for a yukon and tahoe to buy in the last few month (I just brought one with 201k ) I kept coming across a lot of Yukon, Yukon XL , Tahoe and Suburbans with the 5.3 and 6.0 engines, I noticed many esp in the 2000-06 years were ranging in miles from 200k to 400 miles. Unless you suspect your engine is bad , then just keep driving on it. My Guess from all I seen of these for sale is you got another good 100 to 200k out of your engine if you have been taking good care of it. I got lucky with my yukon, the guy who sold it to me, had over haul the transmission and transfer case at 193k miles 2 years ago and only put 8k.. I am going think the transmissions with ones that did a lot of hauling go way before the engines though.
sorry I edit this, I got confused about the miles, he did the transmission at 193k and replace the ac system at 177