Remove the rear cover and make sure the bolt (#K123 at the
diagram) is not loose.
If there is nothing wrong then the rear bearing is the culprit. The easiest way to replace this bearing is to replace the outer tub. Very likely the inner tub spider is bad too.
This a complicated procedure that normally requires some special tools. The top and front panels have to be removed.
If you do not feel comfortable to do it yourself then you can order the parts and hire a pro to do it for you.
- The outer tub assembly
Part number: AJQ35154206 
- The spider
Part number: 4434ER0002A 
Here are the breakdown diagrams and
Parts for LG WM2101HW Washer - AppliancePartsPros.com
Gene.