'99 prerunner issues

My lady friend has a 99 prerunner with 130k and lately she has been having some issues with it.

1. The tranny has started to slip. She has a trusted independent mechanic who says she needs a flush and refill on the tranny fluid. This mechanic has taken care of her truck for the past 5 years and has fixed a few items for a few dollars that the dealers has said would cost hundreds. She has taken it to the dealer for the first 5ish years so lets say for the first 60k miles. I don't know what they or her trusted mechanic has done or not done to the truck. We took it for a drive on the interstate for a few hours and I did notice it slipping on the interstate especially on hills but other times as well. Do you all think the tranny fluid should be changed and flushed or just drain and refill with some lucas additive that I have read on some of the other threads on here? I have not checked the level but it is one of the things I want to do. Would it be worth the $400 for a flush or would she be better off having the tranny rebuild? What is the general cost of a rebuild to fix it? She has never done any towing or heavy hauling with it. Mostly just to and from work and the occasional household project.

1b. I have a 2005 taco, 3" lift 33" tires ,with 38k, when should I change the tranny fluid in it to keep it good for 200k+?

2. A few times when she steps on the gas to pass on the interstate at 70+ mph it sounds like there is a pretty good poping\backfire noise coming from what sounds like the front passenger side of the engine and once on he return trip it sounded like a backfire from the exhaust. One thing I do need to do is put some new spark plugs in the taco. She doesn't want to put a lot of money in to her unless she can't drive it for a long time. She is strapped for cash but I could do a lot of the basics for her.

C. Any idea how much a clean rear bumper would cost? Hers is rusting where the tow-ball should go and it bothers her more than any of the other issues that worry me. I don't want her stranded someplace.

Thanks for all the help.

 

Chris, don't gt me wrong, I value your comments on here, some of your write ups have been a big help for the stuff I have done on my 2005. Being an engineer (electrical) I have to ask why on the flush compared to just a change of the fluid and what do you think is happening.

Thanks man.