Before you pay someone to work on it, try some of the easy stuff, like cleaning the throttle body, giving it a basic tuneup, etc. Stumbling is not generally a problem with the engine block, but rather a fuel or spark delivery issue. If you don't fix the problem now, you will still have it after swapping in the new block and will have wasted a ton of money in the process. The 4.0L is a very tough engine and has proven time and time again that it can go way beyond 200k. And lifting the truck will not kill your engine. Ever.