Earnhardt's three Hendrick Motorsports team-mates are first, second and third in the Chase standings, with at least one win each, while Earnhardt is 24th in the points without a win.
He split crew chief Tony Eury Jr in May this year following a disappointing start to the season, but his results have failed to improve with McGrew.
"We're happy to announce that Lance is going to be the crew chief so we can move forward planning what changes, if any, we might want to make to the team during the off-season and prepare ourselves for next year," said Earnhardt.
"It's something I'm really excited about. Lance has done a great job, he's a really good guy. We have a good friendship that started many years ago. Since we've worked together we've gotten along great and I enjoy it.
"He has built a few new cars for us and every time we take a new car to the track it is really fast. I can see a lot of improvements in the team and the impact Lance is making, without looking at the statistics and luck we've had lately."
McGrew believes the team is heading in the right direction and hopes to establish some momentum before the end of the season.
"I've been in this sport for 17 years," he said. "Of course everybody wants overnight success. When given this kind of opportunity to work with Junior and to work with the best organisation, in my opinion in the world - there are four crew chiefs that get that opportunity and I am lucky enough to be one of them.
"We have 550 team-mates at Hendrick that bleed sweat to make this #88 run as successfully as it is supposed to, and we're heading in that direction. This is the time of year where everybody is looking for direction and hoping you can get a lot of momentum. This is what we're trying to do by setting the record that I am going to be with the team next year and getting everybody pulling in the same direction."
Earnhardt admitted that his confidence has been low this year and is eager to end the season with some good results.
"I think it comes down to the right attitude and showing up feeling like you're the team to beat. I can't speak for everybody on the team, but my confidence was pretty down earlier. It's gotten better. It's still nowhere near where I want it to be, but every week we seem to improve on that.
"I'm really impatient about it but it's just a matter of time. If we can string one or two solid serious runs together before the end of the season, I think it would do a world of good for us."
Burton, Mears get new crew chiefsBarca seek to retain Marquez
