can anyone tell me if its a good Idea?
 

So I live in Michigan where the rust ruins all. But a few weeks back I picked up a 99 Jimmy that is rust free but it does have 250,000 mi. and had lost it's rear ring and pinion, well call it a blessing or I'm hoping very good luck, but a buddy of mine just rolled his 03 Envoy it has very low miles and if I'm correct all the parts I could want for my Jimmy. So my question is do most of the mechanical parts of these two change over? I've been searching and it looks like I can throw the differentials from the one into the other which will instantly make the 300$ cost of the Envoy worth while but can I or more to the point should I use the 4.2L vortex or just rebuild my 4.3 what other parts are worth holding on to for future break downs I do use my toys for what there made for lol Thanks for taking the time to read this and I appreciate any and all advice.

I believe you are talking about the larger Trailblazer body style Envoy since you mentioned the 4.2L vs the 4.3L in which case nothing is interchangeable including the rear axle (its to wide). If you are talking about the GMC Jimmy Envoy then yes most parts are interchangeable since the Envoy version of that model is just cosmetics.

Jimmy envoy was 98-00, your buddy has the later model. Not interchangable.