How can I force SQL Server to use a Parallelism and Merge Join operation vs. Nested Loop

I have a stored proc that's been running great for months and now suddenly is chosing a really crummy plan everytime even though when I run the query independently it runs instantly as it's using Parallelism and Merge Join operations and comes back in less than a second.

Here's a link to the proc execution plan: https://skydrive.live.com/redir?resid=3B34923A73A7291!3957

Here's a link to the query execution plan: https://skydrive.live.com/redir?resid=3B34923A73A7291!3958

Here's the actual query:

declare @StartDate datetime = '5/1/2013'
declare @EndDate datetime = '6/1/2013';

set tran isolation level read uncommitted;

with ProviderDealers as (
       select distinct wm.[Date],
              wm.DealerCode
       from WebsiteMetric wm
              left join Dealer d on wm.DealerCode = d.DealerCode
              left join DistrictLookup dl on d.District = dl.District
       where wm.[Date] >= @StartDate
              and wm.[Date] < @EndDate
              and wm.WebsiteMetricTypeId = 2
              --and (@Provider is null or wm.WebsiteMetricProviderId = @Provider)
              --and (@BusinessCenter is null or dl.BusinessCenter = @BusinessCenter)
              --and    (@District is null or dl.District = @District)
              --and (@DealerCode is null or d.DealerCode = @DealerCode)
),
ProviderWebsiteMetricTotals as (
       select wm.[Date],
              [TotalVisits] = Sum(Visits),
              UniqueVisits = Sum(UniqueVisits),
              FormSubmissions = Sum(FormSubmissions),
              PhoneCallLeads = Sum(PhoneCallLeads),
              UniquePhoneCallLeads = Sum(UniquePhoneLeads),
              ConversionRate = Avg(ConversionRateByFormSubmission),
              TimeOnSite = Avg(TimeOnSite),
              NewInventoryPageViews = Sum(NewInventoryPageViews),
              UsedInventoryPageViews = Sum(UsedInventoryPageViews)
       from WebsiteMetric wm
              join ProviderDealers pd on wm.[Date] = pd.[Date] and wm.DealerCode = pd.DealerCode
       where wm.[Date] >= @StartDate
              and wm.[Date] < @EndDate
              and wm.WebsiteMetricTypeId = 1
       group by wm.[Date]
),
ProviderMetrics as (
select wm.[Date],
       wm.WebsiteMetricProviderId,
       [TotalVisits] = Sum(Visits),
       UniqueVisits = Sum(wm.UniqueVisits),
       FormSubmissions = Sum(wm.FormSubmissions),
       PhoneCallLeads = Sum(wm.PhoneCallLeads),
       UniquePhoneCallLeads = Sum(wm.UniquePhoneLeads),
       ConversionRate = Avg(wm.ConversionRateByFormSubmission),
       TimeOnSite = Avg(wm.TimeOnSite),
       NewInventoryPageViews = Sum(wm.NewInventoryPageViews),
       UsedInventoryPageViews = Sum(wm.UsedInventoryPageViews)
from WebsiteMetric wm
       join Dealer d on wm.DealerCode = d.DealerCode
       join DistrictLookup dl on d.District = dl.District
where wm.[Date] >= @StartDate
       and wm.[Date] < @EndDate
       and wm.WebsiteMetricTypeId = 2
       --and (@Provider is null or wm.WebsiteMetricProviderId = @Provider)
       --and (@BusinessCenter is null or dl.BusinessCenter = @BusinessCenter)
       --and    (@District is null or dl.District = @District)
       --and (@DealerCode is null or d.DealerCode = @DealerCode)
group by wm.[Date], wm.WebsiteMetricProviderId
)
select pm.*,
	ProviderTotalVisits = a.TotalVisits,
	ProviderUniqueVisits = a.UniqueVisits,
	ProviderFormSubmissions = a.FormSubmissions,
	ProviderPhoneCallLeads = a.PhoneCallLeads
from ProviderMetrics pm
	outer apply ( select pwt.TotalVisits, pwt.UniqueVisits, pwt.FormSubmissions, pwt.PhoneCallLeads from ProviderWebsiteMetricTotals pwt where pm.[Date] = pwt.[Date] ) a

How can I get the proc plan to look like the query plan?

Thanks!

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