Lifters going bad or Gone

[njgural]

I was told either one or two lifters are bad, the car makes Clunking noise on the rear left side of the engine, by looking at the engine in front of you. It started about a month ago, I did not drive that much then but now it looks like I'll have a busy Month, My Question is do I have a month or the engin will blow within this month of me driving it. It's freelander 02 with 77.600k, No timing belt done yet, Once I open the engine I think I'll get it all done but then the price goes from about $800 (lifters alone) to $3000 lifters + timing belts/water pump. I'm Debating if it's worth it to do it at all. Next thing I know my Tranny will blow, or rear differantial. I,ve changed the Mounting brackets on rear differential 2 months ago and one already is bad, the car makes Thung noise when shifting from R to D. then as I drive It shakes the whole car, is that a Tranny sign going bad ?