You don't have to but I would recommend it. If your currently mounts are in good shape (which they should be on a 2006) then just go with a JKS MML. If they are not in good shape, replace them with some Brown Dogs. I replaced mine with Brown Dogs and it was super easy.
'98 - 4.0 - HP30 Front, 8.8 Rear, 4.88 -1.25" BL, Brown Dog 1" MML, 3.5" RE springs and crappy shocks - 33" Goodyear Duratrac on Crager soft 8's 15x8 - Toys By Troy front roll cage and custom welded rear roll cage addition.
You dont HAVE to, but it will make your life easier. you wont have to mess with transfer case linkage, or your radiator fan shroud. plus a MML will slightly improve your driveline angles and slightly help out with any vibes you may have. do you run any other lift?
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the slipping would be caused by the shift lever against the floor console. i have the 1" Daystar BL, the JKS MML, and AEV TT - mine popped out of 1st from a stop twice and i could see my console also move when shifting into 3rd and 5th. after a few days of driving, i just pulled the shift boot and looked at the ring that held it in. where there was contact, i saw indentations - i just trimmed it out a little at a time until it didn't rub. without an MML or TT, you can unbolt the shifter shaft and put a slight bend in the shaft.
No, a MML is absolutely not necessary simply because you have installed a body lift.
I dunno why some always insist on pushing a MML for a body lift. #1 a MML is overkill, not inexpensive, and not such an easy thing to install. #2 a MML can alter drivetrain angles in a direction that is not always a good thing. The only thing I'd recommend is simply lowering the fan shroud down an inch to compensate for the radiator, which is mounted to the body, now being 1" higher than is optimal for the engine fan.